Jim & Vickie gave you the most common way without a laser. A flat straight edge (board) across the top of the AC, or highest point. Put a level on the board and keep it level. Holding a board extended out past the side of the coach can sometimes be challenging to hold it level. Once it is on the AC, let it rest and walk towards the edge with the level. Pick up the board and level it. I would use a ladder on the side of the RV and have the measuring person climb the ladder and then extend the tape measure to the ground until it touches. Then, read the lower edge of the board. Use the edge of the board and not the "finished" side. The edge will be straighter.
Now, if you are in the mood for thrilling adventures, find an underpass that is "about" what you think your height is. Pull under. If it does not scrape you are just about under that height. Stop in the middle of the underpass. Climb up on the roof and measure the distance from the roof of the overpass to the highest point on the trailer. Use an accurate calculator and subtract the difference from the height of the overpass. This should give you a more accurate, about, height.
Now, before going on a trip, call all the states department of transportation that you will be passing through that have overpasses of any kind. Ask them if any roads have been resurfaced lately. Also ask if the overpass height has been adjusted for the resurfacing. Most likely they have not because they typically do not resurface under the overpass in most areas. Now you need to watch for that 2 up to 2 inch rise in the surface at the entrance and exit of the overpass
I'm sorry, I just couldn't resist.
A little more serious side. I am always so happy to see an 18 wheeler with a box coming in my direction. I just hope that he traveled the entire highway. The standard height of an 18 wheeler box is 13' 6". How do I know. I asked several and have always gotten the same answer.