While we never got mice, our OEM mattress was moldy from water leakage issue of prior owner. In our case, simply moved in a new air mattress--the big, boxy ones. I've had to patch it a couple of times in the last two years, but it sure has been comfortable, once I added a couple of inches of memory foam.
In an earlier rig, we ordered a custom cut chunk of 4" high density foam. It's not an inexpensive proposition, but it does beat the daylights out of a "custom RV mattress." If you go this route, don't go cheap, but the best density foam you can afford, and do the memory foam trick on top. We once bought the less expensive foam and wound up "sleeping in a trough" when the foam broke down where the bodies lay.
Hey, I appreciate the feedback and good advice. What do you think about a futon mattress. I was going to look into those.