We replaced our memory foam last year for Duvalays and are well worth paying that little extra as they are so much more comfortable than the cheaper ones on the market, we purchased ours at the caravan show at the NEC so had the chance to try all the levels they sell and went for the gold the biggest problem I have now is waking for O/h early cupper. I had a look with regards to your question and they do not appear to be no more of a problem than changing one in the home.We also use a cotton sheet to lie on so not needed to wash the cover as yet
I have bought a topper , not Duvalay though . May be get one later . I then bought a single sheet and stitched it to fit over like a duvet cover . This can then be removed to wash .