Craigslist has a category "parking / storage" that frequently lists warehouses and garages suitable for buses.
The best Craigslist aggregator, IMHO, is Craiggers.com. Right now, when I enter "Prevost" in the search box and choose "RV" from the category list, I get 153 listings all over the country.
Likewise, when I enter "RV" in the search box and choose "parking / storage" from the category list, I get 3,800 listings. You can filter by number of features, like location, to reduce the listings.
Only prolem is, Craiggers only works with Firefox, Chrome, and Safari.