One type of work that may be allowable in some campgrounds/parks is detailing RVs. I've been to several parks in Florida over the winter and private detailers often have business cards posted in the park's clubhouse. Park operators usually allow this activity. I've never used this service so I'm not sure what they charge. I'm sure you'd have to get the blessing of the park operators though.
Of course you'd have to have the tools, be good at it, and have wanting customers. Not sure you'd make a lot of money at it working by yourself.