Our previous 2 trailers we replaced them with Serta's, just like the one on our bed in the house. We never had any problems lifting it. Yea, it's a little heavy, but not so heavy that it can't be lifted by one person. I use a stick to help support it up. I put a little block on the bottom of the bed to catch the stick so we can dig around without it falling.
Our new Outback actually came with a good mattress. Didn't have to change it. But it feels heavier than our Serta's. But it IS comfortable! The bed still does not stay up by itself though.
Get a normal mattress, match the one in your house and sleep well. The problem we had (in the past) with memory foam toppers, is they move around too much, and actually are a PITA. At least the one we had was. We ended up removing it and just got a normal household Serta mattress. Life was much nicer than!