I set the rear table in my 17B as a full time bed. I used the original cushions with a 2" topper, and while it was comfortable, it was a bit too high - blocked the 120V receptacle and, as others mentioned, the folding extension. After my first trip & made some changes:
I replaced the original cushions with a 2" folding gym mat, a tapered 5" to 2" topper that my doctor recommended to help with stomach problems, and a high quality 2" topper over that. Both were trimmed to fit with an electric carving knife. I sewed my own cover for the combined toppers so they act as a single mattress. The advantage of the gym mat is it can be folded back to get at the storage areas and provides more support than the toppers alone. because I sleep with my head at the refrigerator side, there is plenty of room for the table extension & receptacle.
I also removed the table hardware so storage under the bed is easier. I have a couple of laundry baskets for storage. Photos of the bed are on my trailer modifications page
One last note - I found 3/4 sheets at Amazon
- they fit the rear mattress (both standard & my modified version) much better than full size sheets. The ones I purchased have a fitted bottom & a top sheet with elastic in the bottom, making it easier to keep the sheet tucked in. Unfortunately, they no longer seem to be available, and the other one's at Amazon
have lousy ratings. You could try CosyTown Linens
, which makes many different types of 3/4 sheets, but they are expensive.