I don't think we are understanding the exact situation. What you owe on your truck is an irrelevant fact and only serves to muddy up the question because If you put money down on your truck and FCA treats you fair you will get most or all of that money back because the mileage deduction FCA takes should more than be offset by the year of payments you made on your loan.
Let's go through this scenario, where you paid $50,000 for your truck, FCA offers that money back to you. They charge you $0.20 per mile and you have 12,000 miles on your truck. (Mileage deduction of $2,400). Then they give you a tax credit for the taxes you paid. This is what it should look like or something similar.
FCA Offer: $ 50,000
Mileage Deduction: -$ 2,400
Net Offer: $ 47,600
Tax Credit 7.5% of Net: $ 3,570
FCA Reimbursement: $ 51,170
Loan Balance: $ 40,000
Check to customer: $ 11,170
Now, if you buy a new truck for the same price as you paid for your original truck you would still be in the same situation, where you owe the bank roughly $40,000. My suspicion is that you would be able to find a better deal now than you did a year ago when the 2019 Ram 1500's first came out. As you can see from my scenario above, the loan amount is an irrelevant fact to the question you are asking, "is FCA being fair with their mileage deduction?" We can't help you answer that question until we know (1) how many miles you put on your vehicle and (2) what is the deduction FCA wants to make.
I hope this helps you and clarifies the actual question you are asking and cleans up the thought process a bit when deciding to take FCA's deal.