My 05 Sprinter is 8.64 feet tall, or about 8 feet, 8 inches. I have a Fantastic Fan Vent with a vent cover, but the imporant thing is the Tailgater satellite TV antenna which is 17.5 inches tall, plus the roughly half inch bracket I made for it, so it adds 18 inches. So I have a clearance of about 11 feet, 4 inches.
I do have a CB anenna that will just scrape at exactly 12 feet. So I look for clearances that are at least 12 feet.
I have had to turn back a few times, usually with railroad overpasses in small, rural towns.