There are no measurement police, no one will come around with a tape measure to see if your unit is 30' or 30" 6" or 35' for that matter. If your unit fits in the site then you are good to go so long as it does not stick out into the roadway. I have never seen a NPS cg where the tow vehicle did not have room to pull into the site. Maybe not enough room but close. I had the rangers at Yosemite tell me to park my truck sideways once.
Not that being said, if you cannot get into the site then you are out of luck, no refunds and they will not find you a different site.