This ultra simple tool will ease the task of emptying a box trailer or truck bed, though it probably won't work too well on trucks with wheel arches in the bed. It turns your trailer into a cheapo dump truck.
Get any old bit of tough looking grubby wood just long enough to go most of the way across your box trailer, or truck bed. If you have a bit of wood that is too long I think you would be OK to cut it off an inch or so shorter than the inside width of your trailer. This bit of wood is in its third or fourth incarnation it started off as a bit of a pallet, then was a the bottom strut in a temporary frame for my canoe making instructable https://www.instructables.com/id/Simple-canoe/ the top strut would have been better but I think I turned it into a ukulele.
Get some rope long enough to go all the way around the trailer or truck bed with a little to spare.
Drill a hole in either end of your bit of wood and thread the rope through it so that it is about half way.
If you feel like it put some knots in to stop the rope from sliding through. I did but what do I know. If you know how to tie knots properly you may not need holes.
Step 1: Now Use It
Throw the wood bit up the far end from the tailgate, leave the rope dangling just a bit over the end. Try and leave the wood so it is thinnest width up.
Fill the trailer with stuff that needs to go to the dump/green waste depot, or go to the landscape supplier and get some light mulch.
Drop the tailgate and pull on the ropes, most, if not all of your junk/green waste/mulch will come sliding out.
If the rope pulls either under or over the junk and only a small amount comes out throw the wood back in the back and tuck the rope in around the sides and pull again.
Retrieve the tool by pulling on the wood bit. Use the wood bit to scrape any loose leaves etc out.