Allworth, good info especially #2. In rural areas roads get repaved over and over and they can be significantly taller than that height sign indicates, which was usually posted back when God's dog was a puppy. Like you, I add 6" to my actual height, yes I measured twice and then once again until I got the same number three times...
Some places that have tunnels can pose a whole new set of problems, the Blue Ridge Parkway comes to mind. Some of those tunnels are ridiculously low as you get near the right side of the lanes. I see posts just telling folks no problem just stay in the center {over the double yellow} but as a guy who rides motorcycles I take serious offense at that lame advice.
Bottom line, not every RV can safely transit every road. I just wish more folks would realize that and stick to roads that their rigs and skill levels match. Rant off... for now.
:B