I wouldn't bother to change out a truck bed...
The mounts are on the frame, the bed only has the holes. The install instructions that go with the 5th wheel puck kit from Mopar, have very detailed directions & measurements for drilling the holes in the bed. Measure three times, cut once..
After reading the install instructions, I am surprised a dealer wouldn't do it.. Just for the 4 pucks, it's looks like a pretty easy job. I suspect adding the 5th center puck for the gooseneck might involve more structural instructions that could include drilling or welding to the frame... I understand a dealer not wanting to touch that, but just bolting pucks to predrilled holes in the frame, and drilling 4 holes in the bed, at the current labor rates, seems like it would be easy money...
FWW... I just bought a 2023 Ram 3500 Tradesman, 4x4, crew cab, drw, 6.7L HO diesel. It has a year left on the factory warranty, I am the first registered owner, and I picked it up with 36 miles on the odometer. That's not a miss print... it has thirty-six miles on the odometer.. I paid $59,990. which is $13,260 off the window sticker price. It was a far better deal than any of the other used 2023 RAMs I found within 150 miles. Most were off warranty due to having more than 36K miles. The few under 36K miles, were above $70K...
Let the customizing begin...!