Bear in mind that sometimes the level of the door (when open) is not the same as the level of the 'shelf'. Mine had about a 1.5 inch drop from the shelf to the door. I used some rigid pink Styrofoam to shim up under the mattress on the door and that made a pretty significant difference in comfort. Then I threw the foam mattress topper on it and I sleep like a log now.
We use a 2.5 inch memory foam pad and a mattress cover that goes over foam pad and mattress. We then have our sleeping bags on top of that and we don't feel the split in the mattress at all. Slept great every night last week, except for our neighbors very noisy A/C unit that would sometimes wake us up when it started up.