We found it useful to mark the centerline of the camper on the bottom front side of the camper. Then also mark the centerline of the truck box at several points.
This gives a visual clue for the driver as you can see the marks on the camper and truck box and ensure they're lining up all the way in.
Our camper has only about a half inch clearance each side around the wheel fenders. We've learned it is best to have most of the gap on the driver side and snug fit the passenger side because the back-of-the-camper driver-side has a protrusion that won't clear the tailgate area otherwise.