Not applicable in your place, but in the US individual Walmart stores are allowed to permit parking "large vehicles." It's left completely up to the individual store manager, but in some places it's about the only place one can stop overnight (or a little longer) without fees, and of course you have to ask (before you park) if the local manager is willing.

In a community where the town is "supportive" of a festival, it might be possible to convince a few commercial businesses with a little more parking space than they really use that it would be great "public relations" to let a very few people park on their lots; but that would likely require a "local with an agenda" to do some selling of the idea.

John