I know I'd be unable to use the toilet in mine if I put a 6 gallon water jug in the "shower", looks like yours is the same. I'd put them just inside the door when you come in for the night, move them back outside when it warms up a bit in the am. Don't know that I'd do more then 1 or 2 though, they aren't all that light. Could always get something more manageable like 3 gallon. Might be easier to store somewhat out of the way in the trailer, and get out of the truck bed.
http://www.rei.com/product/605745/reliance-desert-patrol-3-gal
