A few years ago I had an 11.5 foot Caribou truck camper on a 3500 dually.
When I checked in to the Great Falls KOA, they sent an employee on a golf cart to guide me to my site. I took one look at the site, and told the employee "NO!"! There was a low hanging branch that would have scraped the top of the camper.
They gave me another site that did not have any low hanging branches.
IMO, I always have the right to refuse a site that does not fit my rig, whether it be height, width, or length that limits it. I am not a bit hesitant to state "No, this is not acceptable!"
It does not happen very often.