unfortunatly they are legally required to, most people take your position, and the insurance company will do everything it can to avoid following the letter of their contract (that is why government oversite is necessary in some situations)...Like I said, I have advised on several cases like this, and to be honest, make them the offer like I said above, if they dont take it file a lawsuit (normally I would not say self-represent, but this would be an exception)...I have seen 4 cases (out of more than I care to remember) where the insurance attorneys actually let it get in front of a judge...all 4 times after the opening the judges literally asked the ins atty why they were wasting the courts time.
Keep in mind right now is the "psychological attack" of the negotiation...they need to convince you that your truck is only worth what you paid for it or less (after all, you did put miles on it correct), next will come your negligence and actual condition (this will be minimal seeing as your short term of ownership hence contacting the previous owner), after that comes the low-ball non-comparable vehicle offers (your truck compared to an 87 6.9 w/300k on the clock).
right now they are playing on your emotions (not meant to offend, but it is damn near impossible to be totally unswayed by a what happened), mix that with the fact that they are professionals at negotiation, and you have exactly where you are.
Best advice I can give right now....take a step back, read all of the fine print in your insurance contract...there should be a section on resolutions, restitutions, and compensation....focus on just that...because that is your key, and they will do everything to keep you from focusing on it.
As a totally dumb sideline question, you mentioned full coverage, and usually there is also provisions for a rental car while all of this is going on...has this been provided yet? (the only reason I ask is it goes to state of mind of both you and the ins company).
If you want me to have one of the legal teams I work with review your contract PM me and I will send you my contact info