Yeah last time I checked off-road diesel was about 50 cents cheaper than on road diesel. So 50 cents times 100/200 gallons, can save you some money. Especially if you do it regularly. And I totally didn’t check to see if it was worth running in my diesel truck, you have no evidence.
Oh and for when they check, it is usually at DOT weigh stations. If they decide to do a random check or they see something wrong with your truck. They will do an inspection and usually dip a tank. Or at a traffic stop, it is usually only a state patrol that bothers, but if you’re unlucky enough to get pulled over in a semi truck. It is not outside the realm of possibility the officer will decide to dip your tank.