Lots of people run into this issue with the newer trucks. Truck manufacturers and RV manufacturers really need talk to each other and figure this one out because right now you would be hard pressed to buy any of the big three new HD trucks and hook them up to a 5er that sits level.
You have several options.
-Lower the back of the truck with lowering shackles
-Raise the pinbox as high as it will go on the 5er. But need to keep an eye on bed rail clearance. Similarly lower the hitch head if its adjustable on your hitch.
- If none of that works, your only option is to raise the trailer at the axles.