No issues with me having to pay for additional guests coming to your paid camp site and using additional resources or whatever is offered by the camp host??
At state parks in WA State, additional vehicles have to pay $10 a day to park.. It's called the Discovery Pass. You can buy one for the whole year for $30 and use it a million times, or pay $10 a day..
Visitors will use the parks resources, so it's not a free ride and should it be??
If you don't want your 89 year 'young' Father in law to pay the fee, YOU pay for it... Pretty easy.. If you don't want to have to pay a visitor fee., well, you don't have to stay there...
Just the way of the world these days... Private or State run..
Good luck!
Mitch