I made the mistake of not measuring when I picked it up in Dallas. I have to tell you the drive home to El Paso was pucker inducing. When I got home I measured and when hooked up to the trailersaver hitch in the back of my ram 3500 dually, the bedroom AC is right at 13'2.
Hubby measured when we were first hooked up at our pickup date. Our F-250 squatted a little, not enough for air lift spring thingys, and we measured at slightly over 13ft.
I don't know about the best method but while I was hooked up to my truck I jumped on the roof and had my wife hold the bottom of a measuring tape and we measured from the ground to the top of roof right next to the bedroom AC. Then I measured the AC height and added them.
Easiest way is to get on top with a straight board (2x4 will do) long enough to hang over the side of the trailer with it laying on top of the tallest piece of equipment on the roof, drop a tape measure to someone on the ground, you measure up to the bottom of the board. This is the method I use.