We have center marks on the truck and camper. Easier to watch every detail with our standard cab. Wife on blind side back where shower bumps out of 4′ box line. Also have reflectors on inside of “flanks” that rough align with truck bed rails. Also use jacks / lower sequence to final center (tape measure points in back). Pieces of ⅜” conveyor belt dropped vertically next to wheel well as shim. All probably much more than needed and its only time expensed for getting it just right.