2 years ago I bought this
SignatureSleep 10" memory foam mattress from Walmart.com. It is firm enough to support and not sag or collapse like a cheap RV mattress yet soft enough to feel comfy. I just ordered the 12" to replace my home bed's worn out king size. My king already has a 4" topper but sags badly because the spring mattress is worn out. I just can't bring myself to shell out another $1200 to buy another BeautyRest(or whatever) when a memory foam feels great for 1/4 the price.
2 years ago I also bought a SigSleep 6" twin and cut down the width to fit the TT bunk bed. My son appreciates a nice mattress as well. :Z