I've done a lot of research on memory foam because I wanted to get memory foam beds for home and camping. First, memory foam should not be used alone because of the "sink through" issue previously mentioned. Get a topper and put it over the TM mattress or "build" a true memory foam bed which is a combination of the memory foam and two layers of regular foam. The bottom layer of regular foam should be more firm (have a higher ILD number) than the layer immediately under the memory foam. Ideally the two layers of regular foam are (when combined) twice as thick as the layer of memory foam but that usually isn't possible in a camping trailer.
The bed I have at home has 4" of 38 ILD regular foam as a base, then 4" of 35 ILD regular foam and then 4" of 13 ILD, 5-pound density memory foam. The one I have in the trailer has 2" of 38 ILD regular foam, 1" of 35 ILD regular foam and 2" of 13 ILD, 5-pound density memory foam and it is
wonderful. BTW, ILD is a measure of firmness. The number, e.g., 38, refers to the number of pounds of force required to compress the layer of foam 25% of the way or, put another way, to push it down to 75% of its actual thickness. Higher ILD numbers mean more firmness. Firmness is different from density, which is measured by how much a cubic foot of the material weighs in pounds. My advice is to never get memory foam with less than 4-pound density. You will sink through it too easily and it won't be durable enough to last a long time.
Also, only buy memory foam made by one of these companies: Foamex, Novoform, Isoform or TempurPedic. The rest of the stuff out there is foreign-made and comes primarily from China. This is not just a patriotic, "buy American" point. The stuff from China is often made with noxious substances that can out-gas from the memory foam for years. Also, some Chinese factories have been known to add clay particles to their foam mix to make it seem more dense than it really is.
You can get good memory foam made by one of those companies at Overstock.com but you have to know exactly what you are doing as they also sell a lot of crap. The same is true with the big box stores. I'd recommend the following two on-line companies because I've used their products with great success and satisfaction. (I do not work for them or have any connection to them.)
Sleepwarehouse
The Foam Factory
These companies sell memory foam made by Foamex. The Foam Factory also sells regular foam made by Foamex if you want to build a bed like I did. Both companies sell toppers. With The Foam Factory you can specify a custom size for each piece of foam.
Sweet dreams!