Of course the simple answer if you are offended by someones choice to exclude you is to not stay there.
However, getting all upset and spouting about how denying the campground your $25.00 fee will surely make them sorry...makes little sense.
Sorry, but you should step back and think this through...
On the other side of the coin, consider the campgrounds position;
Perhaps in the past, the campground has hosted a rag-tag band of "travelers", or some old hippy bus, or some brewed in the woods homemade contraption.
Maybe the wiring or plumbing was not up to a reasonable code, and when they turned on a few things in their camper, it back fed into the campgrounds electrical system causing damage to other guests campers.
Perhaps the home brewed water and sewer systems in the home made campers allowed black water into the campgrounds fresh water system, making other guests sick?
Insurance companies do have a lot of control over what they will allow. Of course you can do as you wish, but you can also lose your campground/livelihood if someone sues you and you don't have insurance.
Maybe it has nothing to do with the insurance company, but rather the campground is reasonably successful in keeping most of its spots rented to xyz people with xyz types of campers. Along comes a home built or two full of loud, rowdy young people, or a few bad experiences with home builts dumping raw sewage on the ground causes the regular customers to tell the campground that they are going to camp somewhere else if these kind of people are allowed?
If it was your business, or your paycheck, and you were going to lose 90%+ of your income, what would you do?
Campgrounds are a business, and like any business, exist to make a profit by selling goods or services.
If a campground turns away your home built, it likely has very good reason, as your money is just as green as the guy with the RVIA sticker on his camper.
There has to be a good reason, that fits their business and circumstances, or they would soon be bankrupt.
Rob
Waiting for "someday" will leave you on your deathbed wondering why you didn't just rearrange your priorities and enjoy the time you had, instead of waiting for a "better" time to come along...