Camper / Bed Alignment Products - Do they exist?

For those that are saying that there camper does not shift with their given setup are you basing this by measuring or just observation? When I place my camper on my truck I'm measuring it down to a 1/4" to ensure it is in the middle. I only know the camper has shifted because when I measure again it's off by about an 1" or so. I would not be able to tell this by observation because the amount of slippage is fairly minimal.