If you are really talking about campgrounds, not RV parks, many accept only cash up front. Most want exact change. The only time I've been asked for ID has been to prove I am old enough for the senior discount.
What might matter is "minor" by what definition. Across our Federal system of each state making their own laws, we have legal barriers at a lot of different ages: now 21 for drinking, 18 for voting and selective service, and 14, 15, 16, 17, or 18 for legal accountability, driving, marriage, compulsory school attendance and ability to "consent."
If you are going to Florida, check the laws and park regulations for Florida, Georgia, the Carolinas. In most cases, if you are old enough to have a driver's license, you are old enough to be alone in the campground.
You can run into problems, however, when a group includes minors. In an Oklahoma state park, I watched a group of young men and boys expelled from a campground because they were drinking in a group that included minors for the purpose of drinking, and child services had to come out to take one of the minors because he was below an age for personal responsibility, and no adult in the group was qualified to be his guardian.