Looking for mattress replacement for Flagstaff pop up

I went with the original mattress and a three inch memory foam topper (Amazon)
that I cut to size. Buy I considered new mattress. If the foam hadn't worked out then my solution was going to be an air mattress with the foam topper.
I seem to recall that I called the factory parts people and was quoted $125
for new mattress's ea. (At 4" thick)