A few years ago we towed our 2015 FunFinder F210UDS under an overpass and it wasn't until I looked back that I realized I didn't know how close it was to the top of our A/C. So when we got home I built a super simple measuring device that would help prevent us from having to stop and shovel up roof parts.
It's just 2 pieces of 1x4 pine carefully screwed together at a 90 degree angle. The horizontal piece has to be long enough to reach the middle of the trailer's roof, so maybe 5ft. The vertical piece should be long enough to comfortably hold against the side of the trailer, an 8ft piece should do the trick. Make a mark on the vertical piece exactly 6ft down from the bottom of the horizontal piece. Rest the contraption gently atop the high point of your trailer, quite likely the A/C housing. Add 6ft to the distance to the ground from that mark you made and, Voilà!, you have your trailer's height. Sure beats finding out the hard way.