Just as you would for any commercial vehicle, you are required to keep the receipts with the vehicle at ALL times to the end of that month. This is where to roving "weight men" and scale houses come into play.
If they can smell french fries or straight alcohol, you have to have already paid the tax and have the receipt to prove it. Same goes for farm vehicles that buy untaxed fuel but on occasion drive on public roads...they have to pay a tax.
You cannot get around taxes.