I share your concerns as I am about to buy a hitch for my 2020 GMC 3500 Denali that has the same 21-inch-deep bed. I will be towing a Grand Design Reflection 29RS. The challenge is striking a balance with cab clearances in a 90-degree turn, tailgate clearance, bed side rail clearance and keep the trailer near level while towing. My comments below are based on measurements, not visual observation.
The kingpin on a GD29RS is a Lippert 1621, I suspect the GD303 uses the same kingpin. In my case, the problem is that at a kingpin height needed to allow near level towing it appears the king pin frame would be very close to rubbing the tail gate, and in a 90-degree turn rubbing the bed side rail(s) along with having inadequate cab clearance. Using a sliding hitch would alleviate the cab clearance problem but creates a need to remove or replace the tail gate. If the tailgate is removed the tailgate camera must be unplugged and when unplugged the other cameras will not function. [Lesson learned if you close the tail gate and do not have clearance to close the mid gate, you cannot open the tailgate; the midgate must be closed for the electric tailgate releases to work. There is no manual release.]
The distance from the middle of the puck system to the cab is 43 inches and even setting a B&W Companion hitch back three inches, it is still short of the 48” clearance needed to make a 90 degree turn without cab to trailer contact.
From a leveling perspective, the bottom of the truck bed unloaded is 38” with the side rail top at 59”. On a 29RS the king pin slide is approximately 49” high at level (unloaded) which makes the gap from bottom of bed to king pin slide is 11”. Using the B&W minimum height of 17”, there would need to be approximately 6” of suspension squat needed when the trailer is lowered onto the hitch to achieve near level towing. That level of squat may or may not be achievable.
I am leaning toward replacing the 1621 kingpin with a Reese Sidewinder Airborne and purchasing a Curt A series hitch. The Sidewinder will provide the clearances for the cab, side rails and probably the tailgate although the midgate may need to be open. It will raise the nose of the trailer by three additional inches so I expect the B&W hitch would be too high even at its lowest 17” setting to achieve level towing. The Curt hitch adjusts in a 13-17” range which should allow room to lower the kingpin sufficiently to level the trailer. The Sidewinder does require a wedge to restrict turning at the fifth wheel, so the pivot point moves back to the pin box frame at the trailer.
If anyone can give me insight on how much suspension squat might occur when lowering the trailer to the truck, I would appreciate it.
