We are back home safe and sound and have given a lot of thought to our trip through NY city and done a bit of research as well.
First, we also have the Rand McNally RVND 7720 and had it programmed for our 12'-6" height which was per the motorhome spec sheet. The extra 2" comes from the sat antenna which is a bit taller so we are actually 12'-8" per my measurements.
The RVND did indeed warn for low clearance but it really doesn't have anything to do with the height programmed into the GPS. It alarms on any low clearance nearby, could be 13'-6' or more or could even be on another nearby underpass. This makes the low clearance alarm feature on the RVND 7720 nearly useless in my opinion.
Second, according to my research, the height of the typical 18 wheeler tractor trailer truck is anywhere between 13'-0" and 13'-6" or higher. During our tour through the low clearance areas of NY there were 18 wheeler whizzing all around on both sides. My only conclusion is that there is a lot of fudge factor, in this area at least.
Bottom line, my conclusion is that although there indeed some low clearance areas that would get us, if we're on the road with 18 wheel tractor trailer trucks all around we won't have any clearance problems. So I am back to my original question, worded slightly different, ""Has anyone every hit a bridge or overpass in a motorhome on interstate highways or other roadways regularly used by tractor trailer trucks?" My guess is the answer is no.
Bill & Gayle
2005 HR Endeavor DST
2014 Jeep Grand Cherokee