We were out camping one weekend and drove past a mattress shop that had a sale on. Bought a standard length memory foam mattress for under $200 (IIRC) and shortened it with a bread knife on a picnic table in a CG. No idea of the brand. That was 7+ seasons and 3 TTs ago and it has been an excellent mattress and has provided some of the best sleeps compared to our fancy dancy megabuck memory foam mattress in the house. If I were buying it today, I would have extended the lid on the underbed storage an inch or two and cut the mattress longer.
One thing about the memory foam mattress is that it is heavier than an RV mattress. I had to relocate the mounting points for the gas shocks in our underbed storage so the lid would stay open.
The mattresses that come in RVs are torture devices... Memory foam toppers are a waste of money.