We've had a 1-1/2" thick memory foam topper from Walmart for 4+ years. It floats on top of the 4" thick hinged mattress that came with our boat. Our guest room doesn't have one.
Here's my experience:
1) I have womanly hips and sleep on my side. On the occasion we have to sleep in the guest room, I don't sleep well at all! My spine curves without the extra padding for support. The 1-1/2" of foam is just enough.
2) Dave prefers sleeping without the foam because it's much much cooler. I agree but I can't give up the extra thickness.
3) Our bed is oriented sideways and there's little headroom
. Making the bed is like wrestling a giant octopus. We have to flip the back mattress half up first, which makes the topper slide toward the center and gather in the mattress hinge. It takes a coordinated jump on all fours while pulling the topper forward with the fitted sheet in place to make it lay right.
4) Whatever you choose, make sure to consider your your available headroom