To take this further, my current adapter has no markings but looks similar to the C5G offset offerings but this is an old one with no cushion.
The offset appears to be about 7", and the height is 15" from top flange face to very bottom of the goose hitch.
the interference is indeed with the pinbox itself wanting to make contact with the truck bed, in which the extension is just making the problem worse.
What I am not familiar with is what normal, acceptable clearances should be? I am used to hauling trailers that have a mile of clearance.
I measured from the center of goose hitch to the corner of the bumber and that is 66", and the distance from where my actual RV king pin to camper body is 72". That means "technically" the trailer would clear the corner of the truck by 6" without the offset.
I measured the distance from my bed floor to the top of rail at 22" and I measured the box height at the swing radius WITH the goose adapter at about 20". That is consistent with my experience when pulling. I know I can't turn very sharp without contact.
I roughly measured the distance where RV to top of bedrails would be at about 6".
From the looks of all of this, it would see if rear bumper clearance is acceptable without offset, I would probably be running an adapter without offset.
As well, I noted that the pin box has two height adjustable holes. It is at the lowest hole which lifts the camper up, and I know for certain it is a bit hiked up so moving the box to the upper set of holes should help level the camper if I can maintain acceptable bedrail clearance.
As far as my goose hitch, it is in a very optimal and factory mount position. I noticed where some 5th plates actually put the king pin behind the axle and that is not a good idea! Aside from playing with campers, I spec professional rigged loads so I feel good about my goose location.