Guess the big question here is whether they charge that for all new residents. Landlords are not permitted to discriminate based on protected factors, including race, age, religion, and sexual orientation. If you’re being asked for a higher entry fee than someone else on the basis of a protected factor, the park may be violating fair housing laws. If, on the other hand, they charge that amount to all new residents, that is not a violation.
That does seem quite high if it is an entry fee. Is it possible it is the down payment on the mobile home? Or could it be the purchase price for the lot? If that is the case, then the amount is not outrageous. If it is only a one-time entry fee, then I would want in writing what is the exact purpose of the fee. I may also be interested in asking some of the newer residents if they paid that as well.