My solution was to buy an RV pump, some hose fittings, stick them in a box, and wire it all up to jumper cables.
Some important considerations:
1. I tried using a 12v accessory plug. The current draw from the pump first blew a fuse, and when I put foil over the fuse it melted the spring inside. That's why I opted to use jumper cables instead.
2. FUSE IT! Make sure you put a fuse in front of the switch, especially since this clips directly to the battery!
3. I just cut up old jumper cables for the battery clips.
4. Use quick release connectors on the pressurized side. You don't want to be trying to screw down a hose in 45 degree rain.
5. Don't use quick release connectors for the feeder because it only takes a tiny air leak to ruin a feeder line.
6. The line going into the water can MUST be stiff and straight, otherwise it just curls up inside the can and won't reach the water.
7. I got that awesome little nozzle at Home Depot. Use It! It's surprisingly efficient. I'm amazed at how much washing I can accomplish with just 7 gallons of water!