Couple thoughts.
The OP probably did the right thing. If he would have told this fellow to "go take a hike" the other guy might have become defensive and who knows what might have happened. It's possible he was a scam artist, or a drug addict, or who knows what. Either way, that person was satisfied and left the OP alone after that (receipt or not).
Next thought:
In Indiana when the State Parks and State Recreational Parks are out of season (November through March 31), anyone can go into the parks without paying an entrance fee (no one is at the gate) and you can just pick any spot to pull in and park and camp (in the campground). There's no one to pay as the campground gate houses are closed up also.
BUT ... before dark, you better believe someone from DNR will be knocking on your door to collect. BUT, BUT... they have on DNR uniforms and clearly are driving DNR vehicles. Someone not dressed in a DNR uniform or driving a DNR vehicle, I'd probably get on my phone and call 911 immediately, or have THEM get the police to run me out! If the police showed up then, I'd gladly pay the fee then (If the guy was not in uniform, because it would be quite obvious he is DNR, and I'm sure the police would understand also).
By the way, there is no water running in the campgrounds during off season, but the camping rates are dirt cheap. They do provide electricity. It's a great way to do a modified boom dock, (If you like cold weather camping).
By the way, in Indiana (don't know about other states), if an officer pulls you over driving an unmarked car, he has to be in uniform, and you have the right to call for a second police officer. If they do not let you do this (run!)