Just a suggestion to help you make sure you always have plenty of possible customers. If you can't figure out something on the internet, focus on what kind of services would RV-ers need? If you can find something that you can do that they need, you will be literally surrounded with possible customers.
As mentioned Dog Sitting or walking, barbering, RV repair, computer repair or lessons, design re-work just to name a few. Make up a sign, hang it on a planter hook in the front of the campsite and wait for the customers. If ya get bored, walk the loops passing out flyers.
Good Luck!