We use mattress topper from Kohls and have for the past 3+ years and 300+ nights of camping. Between a sale (Black Friday, maybe) and Kohl's Cash, it went from being a couple of hundred dollar purchase to much less. I think it is a 4" memory foam mattress + pillow top.
It has made it from terrible to OK/bearable. For weekend trips, I would sleep as well as I did at home- likely better. But once you got longer than a long weekend/short week, it isn't enough.
Other than overall comfort, my chief complaint about it is that I get hot on it. I run fairly hot as is and it amplifies that.
Learn To RV: Fixing That Crappy Bed - Mattress Topper
We JUST ordered a Sleep Number bed since we've been living in our RV for the last 4 months. I'm hopeful that'll help.