Height is very important and you must measure it yourself; don't want to take the top off on some old RR bridge.
I have a trailer shed, so I measured the height to the ceiling, then climbed on top and measured the distance from the top of my highest AC to the ceiling. Subtract and found 13' 1".
You can try to measure from floor to top of highest point, but difficult as it is probably an AC, and you must have a level to measure accurately from the AC top out to the edge of the trailer. If you have no shed, I'd say; pick highest point, measure height to top of trailer at that point, then add distance AC rises above trailer top.
I always allow 3" on measurement errors (mostly theirs); ie if the bridge is only 2" higher than my calculated height, I won't go under it.