Got an 8" foam mattress from Wayfair for my first TT. It ran about $250. It was very comfortable, and would have continued to be comfortable if it was just me.
But, as DH is a big guy and he likes to sit in the side of the bed, it started rounding off on the side from his weight. (It took a few years, but the much more expensive house memory foam mattress is now doing it on his side of the bed as well - you feel like you are going to roll off of the bed.)
So, while I really like those mattresses for me, I didn't want one for the new TT if it would have that tendency to compress on the sides.
The new TT came with a more traditional mattress that was little better than the platform it sits on - really firm. So, I went with a $100 4" Serta gel foam mattress topper from Big Lots.
I was very skeptical, but it did well this season. So, I think keeping the stock RV mattress and getting a gel foam topper is a good place to start to be comfortable. The gel foam can be cut if it is bigger than the RV mattress - the options were twin, full, queen, or king.