I buy upwards of 7500 gallons of offroad diesel per year. (business and farm use. 7800 gallons so far this year) My supplier is very forthcoming with regards to his pricing stratagies. Since they're HIS stratagies and not mine, and I'd not appreciate someone telling the world all of my business dealings, I'll not go too far into specifics. But... It's been explained to me that each and every dealer MAY "discount" off road fuel differently than the next guy. Their cost may vary with handling expenses, volume, cost of their storage facilities, volume purchased by individual customers, ect. It is NOT always "taxed fuel price" less state and/or federal tax amount. Long story short, it might cost them more (or less) per gallon to handle off road fuel that it would "on road" fuel. Cost (or savings) passed on to end user.
In MY case, where I'm buying relatively large quantities, my cost per gallon is SLIGHTLY less than "taxed price" less tax amount.