Question for those with the F-450 and heavy towing.

I have a friend with a 2011 F-450 with a Classy Chassis hauler bed (real nice BTW).
The bed adds about 1000 pounds over stock bed weight.
He has Firestone airbags.

He is a traveler, so the truck is packed with the essentials.
With his trailer hitched up and I'm guessing 3500+ of pin weight, the truck still has the sag, even with 55-60 PSI of air in the bags.

Anyone else experience this?