Whether or not they require a separate water meter for each building, residential unit, or for an RV parked on the property would depend on the tariffs. Tariffs are required for all public utilities. They are the rules that say who, what, where, and how much service costs. Depending on the size of the utility, you may be able to access them online. If this is a small town water company, then the rates and requirements would be found in an ordinance that the town passes.
In any event, I would get a copy and read them to see what rules say that an RV parked on your uncle's property requires a separate meter. The property is not subdivided and an RV is not a permanent structure.
And as long as the bills are being paid for the water used, they likely can't shut off your uncle's water. You are not your uncle.
Your $500 estimate is way low. You haven't considered what are called corporation valve (connection) fees. They can run very high. Those are the fees the utility will charge for opening up the road and making the connection to the main. You will also be billed for a minimum usage per month or quarter (depending on the tariffs). That would be much more than your $5 estimate. Ask your uncle what a minimum bill is.

