Maybe a small steel disc screwed to the floor of the box in the appropriate location for a permanent solution?
Double sided carpet tape, or self adhesive Velcro would also work on a more temporary basis, depending how often you plan on removing the camper.
I’ve also seen a contrasting line painted down the centre of the truck box, with a corresponding line on the front of the camper, but even with the two points lined up, one could still be out of alignment. I like this solution!
FYI, Canucks……The alignment kit is available at Princess Auto for less than $18CDN.