I did not replace my 2012 811 mattress but did add a standard queen size memory foam topper. It was the same dimension as the OEM mattress and all our residential sheets fit it well.
I am still using the same topper in my Host 10 years later, so degradation is not that bad. In the summer, we put it under the mattress since it traps body heat and makes it too warm. In the winter, it gets moved to the top. They are hard and stiff if you lie down in a cold camper until your body heat softens them, so the experience is different compared to regular foam or coil spring. I am not sure I would want the entire mattress to be memory foam, but just a layer of it works well for us.