What the OP experienced can to be expected at a KOA that is 15 miles from a metro area with a population of over 200,000+ (Allentown Bethlehem), 50 miles as the crow flies from Philadelphia, and less than 90 miles from New York City. Add to that one of the last weekends before they close for the season.
BTW, if the 30 people who came to dinner next door were not staying at the KOA and it was not a group function such as a Rally, the management probably had no issue accepting the fee charged for "visitors". That is typically $5-$7 each. When the OP complained all they most likely did was put on a Dog and Pony show. They probably knew exactly what was going on.