There's a fair number of places that have that stipulation where we have stayed. It's been discussed here before, but it is a way for parks to screen their clientele and/or justify their higher prices. Kind of a "no shoes, no shirt, no service" sort of thing.
A reasonable solution, if you are subject to the time requirement, is to communicate with the park, provide them some pictures, and see if they still think you would not meet their standard, albeit arbitrary. I have friends who have done that with their vintage or restored rigs without a problem.
At first blush, it might appear snooty or unfair, but I think a business owner has a right to establish a standard for the clientele he wants to serve, and as customers, we have the right to look elsewhere if we feel his policy is unfair or unreasonable.
I think your choices are probably pretty numerous, regardless.