I've owned a Class C for the past 4 years and have alway's had it stored outside at a storage facility due to HOA restrictions in my subdivision. I've paid about $60 per month and it is in a gated area with security. I haven't covered it but I wish I had and am going to be purchasing a cover soon. I am in Indiana so we do get snow and ice storms. I really haven't had too much of an issue other than the roof vents damaged in a hail storm.
The place I use does regular security checks. I've seen them driving around when I've been out at my RV doing some cleaning or minor maintenance. The only real issue I have had is mice. Two years ago I had one get in the RV and then just this past weekend we had another one. I'm not sure if they actually got into the RV at the storage location or at my house when I had it parked for two days prior to our trip.
My only tip is try to find a location that is not too far away so you can run over and check on it from time to time. We don't get to use ours as often as I'd like due to kids activities so I will stop over every once in a while to check on the RV. Start the engine and exercise the generator. If the storage is too far away then its easy to let too much time go buy.
Also, depending on how strict your HOA is you can usually get away keeping it for a few days. I typically go and get my RV on Wednesday's before a trip so we can clean, stock and do a test of everything before heading out. Then upon return we usually will have it in the driveway for a day or two before taking it back. That way we can clean it good before going back to storage.