OP here. I don't know what my current mattress is - it has a pillow top and most foam mattresses do not. It doesn't sag, though.
I never used the straps on mine, didn't seem to need them. They would probably help keep a foam mattress in place. Foam mattresses are really heavy, though. About 75 lbs. for a 10" and 55 for an 8" for the ones I have found. And that's in the short queen size.
Also, my front wall slopes, and has lots of space between the surface of the mattress and the wall. My mattress is 7" thick, and there are 6-7" of space at the top and about 15" at the bottom, so there is no sandwich effect, but plenty of room for a thicker mattress.
Last thing, I emailed a foam mattress maker that sells an 8" short queen about this issue; they do not recommend any of their products to be used on a murphy bed, RV or otherwise!
So, I don't know what I'm going to do, but this might be of interest to others with murphy bed RVs.
We're about to spend four months in the RV, and that bed sucks! Anybody else have any ideas?