Well, you should be deducting all gas used as a cost of doing business. Since I tap the TM fuel from my truck tank, I take the total on every fillup, deduct the hours used on that fillup, and multiply hours by 3 GPH, then multiply that times the cost per gallon for that fillup (since the price changes so often), and that gives me cost of fuel for the TM. The remainder is fuel used by the truck itself, and that is deducted as an expense for the van. And, yes.....I document every purchase, and record the mileage and machine hours. And I have been keeping those records consistantly every year, incase the IRS wants to know.
If you have already deducted the fuel, I don't think you can double dip and also deduct the taxes....again. But ask a CPA if you really want the answer.