Didnít try the My Pillow topper, but we did buy a 4Ē memory foam topper from Target (donít recall the brand). It was a regular king size, which was a bit larger than our RV king size mattress. I trimmed the excess with an electric knife.
The topper made a huge difference in comfort. The original mattress was pretty much crap. But, it sleeps great with the topper.