I am pretty sure you measure just the camper itself. For instance, my old camper was 21'6" hitch to bumper but the actual camper was around 18' i bought a cover that size fit between 18-21 and it fit like a glove with a little extra material in the front which tucks in the front and the straps lock it down. The brand of covers you are looking at usually give you instructions on how to measure before ordering.
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