If you carefully read the mattress descriptions, you'll find that the premium ones have the top 3" layer of quality memory foam and the foam below is actually just to support the upper layer (some have three layers). If your bed is of decent quality, a 3" topper will work well (and is cheaper). I have a Sleep Number at home and added a 3" topper. It's great. For the Alpine I bought the Novaform from Costco that has the air chambers and 3" memory foam top layer. It works well and is almost as comfortable as home.
The new gel memory foam is the latest incarnation. I haven't seen or tried it yet. Notice the posts above are mentioning different types of memory foam configurations, but not the gel memory foam. They are a bit different.
2003 Alpine 40FDTS (400HP)
Long Beach, CA