I've carried a queen-size mattress on my old long trailer. The first attempt was a disaster, in strong winds that flipped it all less than 50 yards from my driveway. The incident basically ruins the hitch and the neck of the trailer. In the end, I did get it to my storage unit more than 3 miles away after repairing the trailer, but the repairs were temporary at best, and I dismantled it and disposed of it.

The picture shows the set-up but with a futon. In this case there were two bins like RubberMaid ones underneath to lift the bed above the wheels. Same principle with the mattress.

Machka and I use inflatable mattresses when car camping. They are good, and we now stack two so we don't have to drop so far down to lie on them.

Much depends on how tight you inflate them as far as comfort goes. Which is the nice part about inflatable mattresses if you have back, hip or other issues.