Don't have a ladder? On a sunny day, park broad side to the sun so the shadow of your A/C can be seen clearly on the ground on a level surface. Measure from the center line of your trailer to the peak of the A/C shadow. Find an item short enough to measure its height (or use a yard stick) and measure it and its shadow. Figure out the angle of the shadow by taking the inverse tangent of the stick height divided by the shadow length. Then simply take the tangent of that angle and multiply it by the length of your trailers shadow. Voila - no ladder needed.