remember your 3500 sits very high in back empty, and that is what headlights were adjusted for.
putting weight on truck is going to cause back end to drop, and headlights will shine higher.
I don't agree with others saying to install air bags to stop it from dropping, I want mine to drop so it rides level.
first night trip with my 990 I took a T20 torx screwdriver with me. open hood, on side of body behind headlight. 2 holes, look in them, only one has a adjustment screw (up and down) There is no left to right adjustment.
I turned my headlights down around 3 full turns. I think CCW was down, but don't take my word for it. pull up with lights shining at a wall.
nobody was flashing me because the headlight were so far in sky it did not bother anybody. Wow, now I can see the road.
now for winter with camper off I will adjust them back up some.