Housing benefit is available to secure the occupation of a person's home by providing help towards the rent that person is liable to pay on it. Where that home is a caravan or mobile home it is possible that a person may have two separate liabilities to pay rent, one for the caravan or mobile home and one for the site on which it stands. Help from benefit is available toward both.
As long as the person has a properly enforceable liability to pay rent for their caravan or mobile home, a claim for benefit would be dealt with in the normal way.
This would be less clear cut if a charge was being made for a pitch fee where the site has no planning permission. It might be that without planning permission the pitch fee would be unenforceable and therefore would not be eligible for housing benefit. If that is the case then no help would be given toward any pitch fee that may be charged.