Might put you over height too depending on the 5th wheel. Mine sits about 13'2" on a stock height truck.
x2, i'm 13'3" at my high point on the trailer (front AC) and trailer & truck are level. Truck is stock.
Also, you want 5" or more bed rail clearance between the top of the truck bed rails to the bottom of the RV over the truck to allow for traveling over short dips & hills. If your RV is nose up your bed rail clearance will be less at the rear of the truck & you may have contact between the RV and bed rails.
The dealer that told you it's possible to overload your RV tires if towing nose up is correct.
Stock 2018 Silverado D/A 3500 LTZ