I've never had the issue with memory foam being hot. But then, we always have had mattress pads between the mattress and bottom sheet. In fact on our home bed, we have an electric mattress pad. That thing is great. Much better than an electric blanket IMO. Even if you don't ever use the electric pad in your TT, it solves the issue of a hot memory foam.
In our own fiver, I decided to skip the memory foam topper and just replace the mattress with a memory foam mattress. Had a local mattress manufacturer make one up. Cost us about $400, so while a bit more expensive than a topper, it is oh so nice to sleep on. Nice thing about having it made locally, and having it made of solid memory foam, is that they can easily make it to any size you specify. Solves the issue, and the cost, of finding special RV sized units in stock. The mattresses they put in most RV's are so bad, toppers aren't really enough to fix the problem. You just get a soft surface with the same lack of support. I think most towns of any size have custom mattress manufacturers.