You can search for storage options in a few dozen major cities, and for any size of rig.
All you have to do is enter your ZIP code in their online search form, and the website will tell you what RV-friendly facilities are available in your area.
After you enter your ZIP code and hit search, this is what the results page will look like.
You can further narrow down your choices by selecting the different unit sizes above the search results. Hopefully this service will expand to include more cities and rural areas in the future.
Note: Remember to check if your RV insurance policy covers your rig during storage before you sign any contract.