Center of gravity isn't near as critical as you think it is. 1lb to heavy, 1" too far back, is not going to matter.
Your truck's COG range is also much wider than you think it is. Read your truck's camper loading information carefully. The picture may show that the COG range begins just in front of the rear fender well and ends just behind it, BUT don't go by the picture as it's not to scale.
What you should find is that the COG range covers most if not the entire length of the truck bed. I haven't checked my current truck but all the previous trucks I've had are like this. As long as the camper weight is within the rated capacity, the COG can be anywhere in the bed without any undue handling issues.