Maybe I'm not correct here as I just don't think that much about height, I've never measured my 5th wheel, I think it is under 13'. But, what I do know is I am not as high as a semi. So, if large trucks are traveling the road, and they clear the bridge, then I have nothing to worry about or even think about. As for roads that are not main roads of travel, I have yet to come across a low bridge - so far. We have a trip to Glacier scheduled for this summer, I already know they don't allow large trailers into the park and have a low clearance at the entrance, so we will not be going in with our trailer. However, no problem at Yellowstone.
It's good to know your height, but I wouldn't slow down on a freeway unless there lots of signs detouring large rigs due to construction or something. I guess you could be concerned at some gas stations, but I haven't seen a low roof in a gas station in years, or at least one I would be comfortable filling up with a trailer attached.