Get caught with dyed, non-taxed fuel in your tanks and the tax man will burn you a new one. I'm not sure exactly how they would find out since we don't do safety or roadside checks, but if they did?
They'll charge you with tax evasion even if you're carrying in separate tanks, if you have the capacity to transfer into the highway vehicle fuel system....and, if there's not some obvious use for the off road fuel.
You could run it in a separate generator tank I would expect? As long as you couldn't transfer from tank to tank.
One way to find it is to check up on the vehicles owned by said farmer. If he cannot show how the fuel is being used, he's gonna mention the "bus owner!"
Has the tax-man called yet? He will! I believe you should leave town this weekend....ride down to...say, Timmonsville. Should be safe from the CIA and IRS down there.