My Newmar is 12' 10". Measured from the top of the satellite antenna. I have encountered 2 bridges 13' 6", one on a major Interstate and one on a side road. I have replaced 2, (18") antennas that have been broken from hitting low bridges. Antennas take a beating on low bridges.
There are around 5,000 bridges under 13' 6" in the US. My Rand McNally RVND 7720 has low clearances programmed into it and I downloaded the low bridges into my Delorme system. Thus far they have done a good job of routing for my needs. On the Delorme system I can look and see all the low bridges Icons on the map.
This is what the clearances are supposed to be:
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