I tried squeezing our 6" full-sized foam/memory foam mattress from Walmart into our 16' Casita
but it was much
too tight a fit. It's interesting that Charlie was able to put a full-size mattress into his SD without cutting it... maybe the bed area is wider in the newer Casitas. Have you measured your bed to see what its dimensions are? Ours was 45" X 75". I ended up cutting the mattress to size with an electric knife, but I think 6" is about the maximum thickness my knife would have been able to cut. (An upholstery shop could cut it for you if you discover it's too big for your electric knife, however, even the 6" mattress was very heavy and difficult to move around... the 8" one would be even more so!)
The mattress cover was removable, so it was easy to re-stitch it to the new size. We love our new mattress!