We have a coachmen freelander, but I currently have a featherbed in the cabover and it works well. The only complaint is that my daughter says the feathers poke through. When it's time to replace it I may go with a down alternative instead.
The nice thing about the featherbed is that you can "squish" it to fit. I do have 2 memory foam toppers on the sorry excuse for a mattress in the back. It took 2 to make it sleepable! I had to cut the corner on those to accommodate the bathroom door opening. I didn't want to use a foam topper in the cabover area because I was afraid it wouldn't be thick enough. The padding up there is very firm. But, the kids never complain, and I've only had to sleep up there once. I think one of those 4" thick foam toppers would do fine though.
