I don't think you'll find anything commercially available to meet your "needs". Personally, I think if your going to use a cover in the winter, your best bet is to buy a quality cover (Goldline has a 5 year warranty but are fairly pricey) and secure it to your RV per the manufacturers instructions.
If you think about it, even if you have a sloped surface, the snow still accumulates if it's a sightly wet snow. A good example of this is the roofs on our homes. Here in Iowa, at least, we have snow on our roofs for several days/weeks after a wet snow.
Days camping (2015)----88 days
Days camping (2016) ---110 days
2016 Wildcat MAXX 28RKX (33' TT), 2007 13' Scamp
2015 Ram Laramie Hemi
30 years RV'ing
12 different RV's