As mentioned, storage lots are probably your best solution. I'd look around and see what their contracts state. Some might put it on their website, especially if they're a national chain. If you sign a contract that doesn't say you can't work on your bus, then they legally can't stop you. Of course, there's probably a clause that prevents you from being a nuisance or messy, so they could try to be vague and hit you with that if one (or both) of you are jerks.
If they don't have the terms easily accesible, give them a call and ask... you don't need to tell them you're going to be doing major work on your bus (they'll probably just say no at that point, but I've seen storage areas that specifically advertised to people looking to work on cars). Just say you're wondering if it's ok if you do 'some' work on it.