I have been an insurance agent for over 35 years, specializing in auto, home and RV insurance (only in California). First, the standard homeowners policy does extend to trailers usually with a maximum value of $2,000-$5,000 depending on the policy. This is limited to a utility trailer, certainly insufficient for our Redwoods. A homeowners policy will never cover a motorhome as motorized vehicles and their parts are excluded.
An RV policy should indicate where the RV is stored. There is no coverage problems or restrictions that I have ever seen in a policy. Just be certain to disclose its garaged location.
Lastly, some companies allow the suspension of collision coverage during storage. You should NEVER suspend comprehensive (other than collision) coverage. This coverage includes fire, theft, vandalism etc.
I am totally unfamiliar with any storage facility providing 3rd party coverage in our area.