Whatever height you get may not be accurate. It all depends on a number of factors. Gear in the trailer? or not. Water tank full or half full or dry? Air pressure in the tires, as well as general condition of the axles. Almost everthing mentioned will lower the trailer and diffrent brands of AC units sit higher than others. Your best bet may be to befriend someone who has access to scaffolding. Then you can park next to it and measure your unit.
I assumed that 12 feet was my limit when I towed, that was the best measurement I could get on my 74 Overlander.
WBCCI #5501 AIR # 49
1978 Argosy 28 foot Motorhome
Wise men talk because they have something to say; fools, because they have to say something. -- Plato