Only once in my life have I had a problem with overhead clearance. In December, 1971, I had a 3/4 ton Chevy with a 10.5' slide-in camper on it, and I was also sick with the flu. I thought maybe a milkshake might make me feel better, so I pulled into the drive through of a brand new, just opened, Dairy Queen. I stopped and looked up, saw I had a little bit of clearance, pulled up to the window, ordered, received, and paid for the milkshake, then started forward and tore the vent cap off my camper refrigerator and took out one of the Daily Queen's overhead florescent lights. Of course, the building had been built level, but it was on a very slight slope, so the overhead clearance became less as you drove forward.