The in-laws are good at bringing only an appetite. When we talk about dream trips out west, they are always in...until the cost is calculated. My family shows little interest due to funds or time. We've had some success with friends coming along in their own RV. And we'll let a child invite a friend occasionally and even will have a overnight B-day party with 4-5 friends.
Back to the issue, I offer to pay for the sites, split the fuel and they cover their food. This usually runs them off...I guess they are looking for a free vacation.