You may want to try one of those jumbo space bags for the topper. It saves a lot of room and allows me to see out the back window! After about 24 hours they do gradually lose their seal, but by then it's time to pull them out anyway. The whole mattress compresses down to a small fraction of its size, so it's a real space saver.
I now also have a 4 inch memory foam mattress, comprised of high density foam plus memory foam for the rear cushion, and it is pretty comfortable all by itself. Since two thirds of your body rest on the rear cushion (assuming you sleep with your head towards the rear door) this works out pretty well. I still use a topper for the upper bunk and will also use it on top of the already cushy four-inch mattress at times.  Guess it depends on how firm of a mattress you like, but light years ahead of sleeping on an air mattress.

Troy

<<I started out w/ 2 single - 2" Thermarest pads to supplement our early
style lower bed & they were ok, & easy to store, but not as comfy as
we would have liked, so got the Costco memory foam ($128+-) which is
about 6" too wide to fit flat, but is heaven!  Decided to not cut to
correct width, & actually like it rolling up slightly on each side for
additional support.  I fold the mattress in quarters & store when
traveling, behind the bench seat, & still have plenty of room for all
our other stuff, then store in our "camping stuff room" in the garage
when not needed.

One thing to note, is that when the mattress is in cold weather, it
sets up rock hard & is harder to fold than in the summer when it is
like goo!  Once you lay it out in cold weather & apply your body heat
to it, it softens up.>>