Details on this will be the telling factor.
The spec sheet (linked in the article) shows different payload max capacity for many different configurations. I'm going to assume it's a 3500 long bed 4x4. What we can't see is the engine in the chassis; believe it or not, you lose payload if you get the "HO" (high output) engine. If he has the HO engine, his max payload is 5590 lbs. If it's the low-power Cummins, it's actually rated higher at 5850 lbs.
The camper dry weight
is 4917 lbs
Given that it can hold 66 gal of water (525 lbs) AND 60 gal of propane (240 lbs), (plus a lot of other personal stuff, etc), plus what looks like a small motorcycle hanging off the back (probably 300 lbs there alone). We don't know exactly how much water and propane were onboard at the time of the event. Given that camper is so large and can hold a LOT of stuff, when you add clothing, cooking pots/pans/ other camping gear such as camp chairs, etc, etc, etc ....
I could EASILY see this camper/gear combo getting to 6000 lbs of true weight or maybe more. I suspect he's over-weight, but by how much would depend upon a few details we can't see in the photo and the article does not tell us.
Part of the truck-owner's complaint will be based on what the dealer told him ... You know how sales guys can be; tell them what they want to hear so you can sell the truck!