Yeah, there is no standard thickness of a queen-size RV matteress. Most OEM RV interspring mattresses will be a cheap mattress about about 5" to 6" thick, while cheap foam mattresses may be only 4" thick. So if you want a thicker memory-foam mattress go for 8" to 10" thick.
I replaced the cheap stock mattress in our TT with a Serta EuroTop mattress from Sam's Club. A EuroTop is an interspring mattress with a layer of memory foam on top of the springs. Great for old folks that want a good interspring mattress but with a soft layer as the top layer of the multi-layer mattress. The RV mattress was 76x60 and our standard-queen-size Serta is 80x60, but it fit okay on the walk-around bed frame in our TT. Our Serta is 12" thick at the sides, and the foam layer on top adds about another inch for the fitted sheet to cover.
However if your RV mattress is a short-size 74" to 76" long, be sure the replacement will fit in the space designed for the mattress. If your mattress source does not have an RV-size (also called short queen size) mattress, then go on-line and you can find them. Here's one from Camping World:
InnerSpace 8-inch RV Luxury Deluxe Reversible Memory Foam Mattress - Short Queen 60" x 75"
Or if you are buying foam mattress, then you can pay less for a standard size, then cut off the extra length.
Short-queen fitted mattress covers and bottom sheets are also available at Camping World and other on-line sources.
Short Queen 300 Thread Count Sheet Set, Taupe