I wanted a vegetable garden, but I don't have a yard. Or much money.
I was inspired partly by a book lent to me by the first person I ever went out on a date with
(almost a year ago already!) and partly by the container gardening class I was brought to on Valentine's day by a different date.
When I first jumped into this new experience of dating I wasn't sure what to expect - but rainwater collection was surely no where in my mind as even the remotest possibility.
We actually don't pay for water in the trailer park, (its included in the rent) and RVs use very little water anyway by their nature.
On the other hand, CA is in a drought (again), one never knows when the next earthquake (or revolution perhaps?) might cut off the municipal supply, and I wanted to start gardening without increasing how much water I used.
Step 1: Rainwater Collection System on My RV Trailer
Since the trailer doesn't really have gutters, I started by applying a thick bead of silicone around the perimeter of the roof, except above the slide and the awning, so that the water will flow to those areas. More silicone on one side of the slideout section, and the awning drains to one side because of how it is tilted.
I built the spouts from layers of aluminum tape so I could shape it precisely the way I needed.
The tape feeds into a funnel (mesh covered to keep out the crap), which goes into PVC pipe.
On the slide side its basically just a long piece of straight pipe.
The awning side was more of a challenge as it has to go around a number of various corners and through narrow spaces to get to the storage barrel on the other side of the house.
Instead of trying to measure and cut and join a whole bunch of short plastic pieces, I used flexible aluminum dryer vent.
Three lengths together made the perfect length. It's supported with a bunch of random stuff, bungees,  rope,  metal bars,  and blocks of wood, taken from my cabinets, shed and scrap pile.
After the first little rain the dryer vent drooped in 2 spots, and I added and redistributed the support. 
At the end  an extra large funnel collects water from both sides , and drains into a 55 gallon water barrel. 
The barrel is used, but thoroughly cleaned and pressure tested, and was purchased locally by a company which specializes in second hand barrels, only about 5 miles from my house. A 55 gallon with a removable top (for adding a spout, and cleaning if it becomes necessary some day) cost me all of $20 (even). They didn't have a spout kit - they said they could order one from the warehouse; but I figured I could get something from the hardware store. The book had warned me I wouldn't be able to, and I should have listened. I ended up ordering one online.
Waiting for it to show up gave me an excuse to be lazy and not work on the system, and fortunately for most of that time it was dry anyway (I did miss some rain, which was rather tragic, but what can you do?).
I finished it just in time for the season's last rain, making it the first time in my life that raining actually made me happy.
Step 2: The Self-Watering Planter Box
There was nothing particularly fancy about the design.
Long straight flat piece of wood, screwed into an upright piece of 2x4, shorter flat piece screwed into the 2x4 at 90 degrees from the first one. Just a plain rectangular box, with no bottom.
I made 2 layers of wood on each side, for strength and stability. Some of the scrap wood wasn't wide enough, so I used two boards, top and bottom, [on the short sides, picture 2]. I put on some particle board shelves on the outside [on the long side, picture 3] cause they look better than the scrap wood I had.
(The half barrel in the pictures is from a hauling job; it is not in use yet.)
I lined the bottom with old carpet (also from a dump run) to protect the plastic which goes above it (the plastic left over from a furniture move done in the rain). [pic 4] Then an old blanket on top, both to protect the plastic and to aid in water wicking.
I placed some pieces of broken concrete as supports, and on top of that a sheet of peg board, which elevates the soil above the water reservoir but still has good drainage.
The theory of a self-watering system is that the water below is accessible (via soil wicking) because the soil in some spots dips into the reservoir, but it does not saturate the soil or plants because the majority of the soil is out of the water. In this way it simulates ground water and keeps the soil just slightly damp at all times, but never soggy.
In my system the peg board does not fill the whole planter, so the central area is above the water, but the soil on both sides can reach down into the water pool. 
The mesh keeps the soil from getting into the reservoir.  it is held in place temporarily with duct tape, but the weight of the soil itself should hold it in place once it is in.
NOTE: In a proper, traditional self-watering system there should be an overflow port and a fill tube. I was aware of this when I built mine, but didn't bother. I figured I could always add them in later if need be, and i was lazy. So far, as you can see in the last pictures, the plants have been doing fantastic with my extremely haphazard way of watering: Every once in a while (maybe once a week) I feel an inch or two into the soil. If it feels dry, I add water from the barrel for some more or less random amount of time, (usually until I happen to remember I left it running).
UPDATE - April 2010
My watering quickly became much more haphazard - closer to once or twice a month. Apparently the large water reservoir system actually worked, because nearly everything survived the summer - and the winter. Even the annuals!
Step 3: The Garden
Being my first attempt ever to grow food, I consider this season practice.
The first thing I planted was a potato. It was originally meant for eating, but it went bad, so I threw it in the compost. Later I noticed stems pushing their way around the plastic cover, and lo and behold the "bad" potato was sprouting. So maybe now I will get a good potato out of it.
My neighbor who gardens had told me even before I built the planter that she had a tomato plant for me. She also gave me a tomato stake; which, incidentally, I had given her about a year ago, having gotten it in a dump run and having no use for it at the time. Apparently she took a couple more than she really needed back then.
She also shared some lettuce seeds and a bean plant.
[Initial planting, pics 1-4]
Later I discovered something else spouting in my compost pile. I have no idea what it is, but I figure if it was there it was probably something I was eating, so I planted it. Up until this point I had spent nothing on my garden (except for the materials for the rain collection), but it was getting late in the season and I finally purchased a few plants from the farmer's market (I got a discount because I work at the market myself).
I got strawberries, and an onion. I also planted a few tiny carrots a neighbor gave me. She intended them to be eaten, but they still had the tops on, so I wanted to know if they might get any bigger. After a few days the tops had wilted and I forgot about them under the mulch. A couple weeks after that they had popped back up bigger than ever!
Most recently I planted the seeds of some grapes I bought at the market, but have yet to see if they will sprout.
The neighborhood cats think the garden is an excellent toilet, but by a fortunate coincidence I happened to get a motion activated sprinkler head in one of my hauling jobs recently, and that has completely solved that problem.
It has been a little over 2 months since I first planted and the rain barrel is almost, but not quite, empty. (I also use the same barrel to water my potted (non-food) native plants at the front of the house).
Step 4: Success! Proof That You Don't Have to Have Any Idea What You're Doing to Grow Your Own Food
Everything is growing much faster than expected, but they all seem to be sharing the space with each other very politely.
Today I did some harvesting and actually cooked a meal based on food I grew myself.
Of course this is a normal part of life for a lot of people, but for someone who has lived in big cities his whole life, it feels pretty amazing.
UPDATE - April 2010
My first tomato plant survived the winter and has produced a couple more (tiny) fruits. I didn't add any water other than from the rain barrel all year, gave away produce, and let the insects have as much as they wanted, because it was more than I could finish anyway.
I now have a grape, and another strawberry plant in the box, and blueberry and a dwarf orange/tangerine tree in the wine barrels. I just planted carrots, cherry tomatoes, more lettuce and beans, basil, sweet pea, and some kind of zucchini or squash or something.
I'll try to remember to take new pictures.
Step 5: 2011 UPDATE
After the first couple of years, the novelty has worn off somewhat, and I don't spend as much time in the garden as I did when this instructable was first published.
However, it continues to do well without any supervision.
Step 6: 2012 Update
This meant I had to replant everything into pots, pack all the soil into buckets, disassemble the planter, and rebuild it all again at the new space.
In the process I discovered that the peg board I had used to keep the soil above the water was too weak once it was wet, and it had mostly collapsed, defeating the self-watering design.
When I rebuilt it, I used solid wood instead, and wrapped it in a plastic garbage bag to increase its water resistance. I also left smaller areas for the soil to reach the bottom, so its more like the EarthBox design I stole the idea from.
When I refilled it with soil, I mixed in the worm compost that I've been feeding 100% of my kitchen scraps to for the past couple years (contrary to what the experts say, I throw in citrus and onion and dairy, and all of it disappears), and then I put in 4 layers of mulch: first a big solid sheet of non-glossy paper, then a bunch of old leaves, then some coco husks, then finally wood chips (all of which I collected for free here and there)
Then I dug throw the mulch and replanted everything.
Thanks to an unusually warm winter, everything has been growing crazy lately!