If the camper is on the truck: Take the measurement from the face of the rear bumper on the pickup to the outside edge of the bumper on the camper, as shown in the following diagram.

If the camper is not mounted on the pickup, follow these three steps: 1. Measure inside the truck bed, from the front inside of the bed to the outside face of the rear bumper. 2. Measure the camper's overall floor length, including the rubber bumpers on the front, to the outside edge of the bumper on the rear. 3. Subtract the two figures: the difference between them is the length of the extension you need.