Yep it's very unlikely that they would ever dip a tank on a private vehicle. I've heard of a few people getting their diesel pickups checked, but it's very uncommon. It's also much harder to do you can't directly dip the tank because of anti-siphon devices. You have to open the petcock on the water separator/fuel filter. So you have to know where that is you have to have a little container to catch the diesel in and you're much more likely to get messy. It is not worth a police officer's trouble really.

I'm definitely put off road diesel in my pickup truck a few times. Mostly when I was short on fuel and needed to get to town. Though I have known a few guys who run it regularly without consequence. Though it was mostly due to convenience opposed to tax invasion. They're already on the ranch there's already diesel fuel there and sometimes the boss is letting you fill up for free or more likely they are the boss.

I definitely wouldn't pull up to an off-road diesel fuel pump and just pump it into my truck. That's a little too obvious.