Ok, I am not an accountant, but with my limited mathematical abilities I did a few quick calculations:
Assuming you get a 4% rate (i'm guessing it would be higher)
And they price it around 25K
You pay 6250 in interest for a loan on the 25K
Total payments 75 payments at 425 per month = 31,875
This does not include whatever is going on with the Mercedes.
My humble suggestion, get out of the dealership and get yourself a loan through a bank/credit union. Then go and negotiate the deal on a truck you like. They are making money on the car as well as on the financing, they will work those numbers any way they can yo get you to bite on the car and financing.