real mattress

OK, I admit it, I'm a softy. The mattress in the TM is not much better than the floor after an hour or so. I'm not sure if I want to try a memory foam topper or a real mattress. I think the topper needs to be at least 2" to have any real effect. Would that be too high for the rear bed , as far as not being able to close the shell? Also, the bed is 60 x 78. That takes the thought of a real mattress out of the equation as I believe they're 60 x 80. Has anyone replace it with an air mattress?