Well this topic is typically a personal preference since some folks swear by using RV covers and others just swear at them. Do a search and you will find both sides of the story......preferences for carports or large garages where space, budgets permit etc. I found a link on this forum (not sure it will work but if not logon and do a search.
http://www.thorforums.com/forums/f17...-not-8372.html
I live in Alberta.....(think North of Montana) if you don't know where Alberta is. So Winter is lots of snow and typically a week at -35F and Summers that can get into 90s F. I decided not to cover (snow is my cover in Winter), but I do check roof seals at least yearly and wash the sidewalls several times yearly. So far the coach still looks pretty good for a 2013. In BC (think North of Washington State) rain I expect is your biggest concern.