Snow white - but snow weight? (How much on roof)

One foot is not the least bit worrysome--or, if it is, then you have major roof structural issues that should be resolved regardless of snow or its lack. I might start being worried enough to remove some snow at, say, four or five feet. The roof membrane material doesn't enter in here, by the way; it's a structural question.

Obviously (I hope), if you are going to actually drive the motorhome somewhere, you do need to clear off the roof for the safety of those traveling behind you.