Try a 5 gal bucket with lid. Fill 1/3 full of water, add a little, and I do mean little laundry soap and a couple pieces of dirty clothes. Pop on the lid. Tuck it in the bouncy corner of your trailer and drive to the next stop. It sloshes while you drive. It is ready to rinse and hang up when you get there.
Total investment: one 5 gallon bucket.