Regardless who is at fault, if they are totaling it they need to give you the fair market value. Obviously, the FMV can be argued. But for them to use a single comparative vehicle with 120K more miles and say the value is the same is a joke. If anything, their own comparative vehicle shows your truck is worth more than 5K. There are different ways to determine FMV. In addition to KBB and NADA, there are companies that provide more tailored results to insurance companies for just this reason.
I can't tell you the best way to fight their estimate. Obviously there is a spectrum from ""polite negotiation" to "Your civil trial date is . . .:" I would find 3 similar vehicles, price and condition, and provide them with a letter to the insurance company stating my vehicle is worth X. In contractual terms, their estimate is merely an offer that you can refuse. In the same vein, you can counter offer with your own estimate.
I think you will also find many of the more sketchy insurance companies are harder to deal with than the established ones - whether it is your own company or the other party.