Many RV and Mobilehoime parks offer RV storage.
Most RV Resorts publish a list of local RV related businesses. It usually can be found on the map of the park they hand out during check in. Or on a bulletin board somewhere in the park
Craigslist you need to keep watching for the small lots to advertise storage. You'll be able to tell from the ad whether it's a big lot or a small independent. A tip is to change the ad list you see in Craigslist to "Gallery" this way you see photo Icons for each ad...it will help you identify the independent lots easier.
http://slo.craigslist.org/search/prk?query=rv