I think you're making this a lot more difficult than it needs to be.
Let the Hagerty adjuster review the truck. They are not trying to screw you.
I have a friend that had the exact situation you have, his squarebody was forced into a curb by a car coming into his lane. Long story short the other driver's insurance wanted to pay something like $1,200. Hagerty looked at the truck, understood the parts values (air ride was damaged) and he was given a check for +/-$2500 with the stipulation that if more damage was found it would be covered. It turns out the frame was damaged and needed to be straightened a bit. An additional claim was filed and they paid for that too.
You're looking at this the wrong way, most people usually complain they can't get an adjuster out to look at their vehicle fast enough, and you're trying to duck from one
Just set up a time you can be there to meet with them.
Once Hagerty gets your truck taken care of, they do the legwork to get their money back from the yahoo that hit you.