This might be the ticket, not cheap but I bet the sleep would be excellent
thermarest dreamtime and put carpet up there under the pads to make keep everything from moving, tons of storage and the bed would be sweet.
When we purchased our camper I saw the 1/2 thickness bed pads that allowed the sleeping bags, etc to be kept up on the bed and with the extra length bed it allows all our bags and I keep two thermarest LE pads up there too. When I am by myself I sometimes use both and it is a dream. What I noticed when we purchased our camper was that the density of the bed foam was the same for both thickness so there was no reason to get the thicker pads. They are firm and no way could you push through to the board with a elbow. I also extended the pull out so we can sleep either direction, when I go camping with my brother we sleep along the truck axis so we don't have to climb over each other in the night.
Yeah I actually did a long pull out for mine too (just finished it) previously there was just the 4' cab over part. I had considered doing a 3" mattress on the top instead of 4" to allow bedding to stay up there but that would definitely require buying foam for the whole thing. Where as right now we're considering just getting some firm foam for the slider part and leaving the normal mattress alone (unless they fell grossly different, then we'd replace the rest).