I always had luck at JC Penney, using their Foam Mattress Toppers. I would avoid using anything that calls itself "memory foam", that tends to be really squishy. The biggest question is where to store the mattress pad while you're using the dinette to eat?
Another thought: I slept on the dinette in our pop-up truck camper. I bought the thickest yoga mat I could find on Amazon (about $17 for a 5/8" thick mat) - it was quite firm and comfy; and rolled back up into a cylinder when done. They are only 22 - 24" wide, so you'd want two....
Ed, Deb, and 2 dogs
Looking for a small Class C!