In a previous camper, we tried a mattress topper and it turned out to be the biggest pain the "back side" ever. It kept sliding around, and fit uneven. It was horrible and we paid a hefty price for it.
We got smart then and threw it out and just bought a brand new mattress that was comfortable. And we never looked back. The next camper had a queen bed and a really thin mattress. We just bit the bullet and got a good mattress and was done with it.
Our current camper, actually came with a really good mattress. Five years now and it's still doing great! Factory installed! No complaints.
Advise: throw out your current mattress and just replace it with something that 100% comfortable. After all, you spend 1/3rd of your life on it! That's too much time out of your life to be uncomfortable!