OMG YES! When we first got our RV, we thought the mattress was actually pretty decent. That lasted about a month before it was super flat. We decided to try Mattress Insider's 8" Luxury Gel Foam RV Mattress w/Organic Cotton (our king is 72" x 80"). LOVE IT!!! It has the option of flipping it over for a firmer feel and that's what we use. We've slept on it every night for well over a year and a half and it's still as awesome as the day we got it! They have every imaginable RV size and can make custom sizes.
We liked it so much, we got the 5.5" queen for the guest bed.
If you use our LINK you will get 5% off your purchase!