You deduct the fuel cost as a business expense (at your full tax rate).
Then get the credit for the off-road tax. That adds to your NEXT year's income (because of when you received the credit), because you deducted the fuel cost already (if you didn't deduct the fuel cost in the first place, then this credit wouldn't be income).
If your full tax rate is 25%, then you end up with 75% of the credit in your pocket (100% now, but then you pay back the 25% next year, unless you pay quarterly). You still come out ahead.
Yes, you deduct the full cost of the fuel to the business. The business expense deduction is based on how much the fuel costs.
The credit for off-road use is to refund you the amount you paid up-front towards road tax. That number is based on gallons, not dollars spent. Because of this disparity, there's no way to get around entering all these numbers.