At the front of the RV, measure to the transition point of the flat surface to where the curvature begins (and the rubber roof liner ends). If the width of the RV starts to narrow in the front, then only measure the rooftop portion of constant, uniform width. The back of the RV is usually more "squared-off" instead of curved. Therefore the flat rooftop surface is more easily defined for measuring. If the back of the RV has a ladder then only measure to the ladder.