My old trailer had a double bed, but enough room to squeeze in a queen. I got a 2X10 (which was the difference between the double and queen) and used that to widen my bed platform. I used a couple pieces of 2x12 as legs to bolster the bed to the right height.
Imagine the 2x12 as a bench, with mattress on top. Its something simple you can throw in the bed of the truck when you don't need it.