I've yet to see a chain store bed that didn't bend, bow, or crack. I've sorted out a few with the welder and some judicious additions.
If you are happy with the standard 1200 trailer width. I would look at knocking up a light tube frame that lay upside down in the trailer, with a leg in each corner. Slats across the top of the frame. A bit of innovation may be needed to accomodate any necessary extra length.
Even as a DIY, the materials would cost more than something from the chain store - which tells you how crappy something from the chain store is.
Don't own an inflatable double mattress, but have seen one cheapy in use, and was surprised at how good it seemed to be.