I've had pretty good luck with memory foam mattresses from Amazon.com and Walmart.com. Amazon does have 3/4 and RV mattress sizes listed. Just look for something with many good reviews.
When we bought our TT, wife ordered a 10" or 12" standard queen memory foam from Walmart without realizing how tall it would be. We used it for a couple of years, but recently replaced it with a shorter (in height and length) RV queen to regain the space at the foot of the bed. Both have been good mattresses and were comparatively inexpensive.
Current RV Information: 2018 Forrest River Salem Hemisphere 282RK
Previous RVs: 2004 Fleetwood Bounder 32W; 1999 Four Winds Five Thousand 21RB; 1986 Allegro Bay 27'