First, you should probably start out with your rear tires at the max 80 psi. That is measured with a cold tire, meaning the truck has been sitting in the shade for at least a few hours. You can drop down the front tire pressure to about 70 psi to improve the ride, but the camper weight is all on the rear tires and you need full pressure to achieve the maximum load capability. Under inflated tires will run hot, build excessive pressure and are prone to failure.
Next, expect the tire pressure to increase substantially under load especially at high speed on a hot day. If you start at 80 psi, you could easily see the pressure rise to about 90-95 psi. This is the way the tires are designed to operate and never reduce the pressure because it has built up during use. Again, the pressure should only be set with the tires cold. Even a few miles heading the gas station can throw off the setting.