Thanks to all for the replies!
We bought a Simmons spring mattress today. According to the sales guy, "you can't go wrong with a Simmons". :-)
We opted against memory foam or pillow top as both sagged after a few months in our past experiences. We have a Tempurpedic mattress at home and absolutely love it (my wife's back pain is gone after switching to that mattress; unfortunately it came back after sleeping on the crappy RV mattress), but didn't want to plunk down the money to put a Tempurpedic mattress in the RV. Good old spring technology will hopefully save the day for us.
Jorn