We bought an RV queen sized memory foam mattress years ago for our motorhome from bedinabox.com and got a good deal at the time. More recently though they have been pricey.
Our most recent purchase for our travel trailer was a full queen from walmart.com.
We kept the motorhome for a couple of years after replacing the mattress and had no complaints. We've had the travel trailer for about 10 months and have had no complaints with the mattress we put in it either.
We replaced the "RV queen" in the travel trailer with a standard queen. I pulled the plywood section out and used it as a template to cut a longer plywood piece. The only issue, and this would have been an issue even if we hadn't changed sizes, is that the memory foam is much heavier than the thing they tried to call a mattress the manufacturer shipped the trailer with. This means the gas struts aren't strong enough to hold up the plywood to access the under bed storage. We could replace the struts with stronger ones and may at some point but for now I cut a length of 1x2 to support it.
2018 Forrest River Salem Hemisphere 282RK - 2017 RAM 1500 TV
Previous RVs and TOADS
2004 Fleetwood Bounder 32W on WH W20
2000 Four Winds 5000 21RB
1986 27' Allegro
TOADS
2005 Ford Ranger XLT 2WD
2004 Suzuki Aerio
1988 Chevrolet Sprint