Pickup trucks are different than other TVs such as SUVs. Most pickups are not level when empty. The bed is higher than the front. This is because pickups are designed to carry a substantial load in the bed. SUVs for example are not... Their payload carrying design generally is for a more evenly distrbuted load. That is why they sit more level when empty.
So a pickup sagging in the rear compared to empty is not necessairly cause for concern. Heck my one ton dually drops a few inches when loaded up. It is designed to. As others have said, the front axle weight is the key. You want to maintain at least as much weight on the front axle when hitched up as what it has unhitched. Different vehicles have a variety of measurements to replicate actual scaled weights for this. Read your manual to see what yours says. Or take it to the scales and get the exact numbers.