If you want to know where the COG will end up, pick a common point of contact to measure from.
Let's say the COG is labeled 48" back from the front wall of the camper. Add the thickness of the bumpers, say 3", which makes it 51" back from where the bumpers on the camper contact the front wall of your truck's bed.
So you would measure from the front wall of your truck's bed back 51", and that's where your COG would lie in the truck bed.
I believe that Lance specifies their COG a bit differently. They measure from the "contact point at the rear of the bed" forward, or something like that.
Ultimately COG is not all that critical. Even a 5000lb camper with the COG 6" behind the rear axle of the truck will only have the effect of a trailer with 500lbs of tongue weight.