I agree with your premise MJReynolds, to me it does seem to amount to an illegal search. The FED DOT does have laws concerning 'fuel inspections', they have been doing them for years. I think if you refuse, they will red tag your truck, meaning it is to be parked, towed, and plates suspended until they get their test - or at least that's their threat to get you to allow them to test your fuel.
I have been waived into a couple of those DOT checks in the past couple of years. The go over your truck for all the safety stuff. When it came to the fuel question, I keep about a month's worth of fuel receipts in my truck, I also keep a mileage/maintenance/fuel log, so when they asked me about my fuel I produced the receipts, that satisfied them, no dip.
The fellow ahead of me got his factory tank dipped AND his in-bed tank dipped. I have seen them dip every diesel tank coming out of Mexico intothe US when I have been at the border crossing. Since I do not cross the border, I have not gone through that process.
I don't use the red fuel myself, not even in my Bobcat or tractors. I have a large tank on my place for my equipment, they give me a good discount on deliveries, fuel costs are lower than the red stuff at the pump,and they put in ULSD. When I am traveling I pay whatever the pump price is, rather NOT have any risk of the issues associated with off road fuel.
As I see it, the money/taxes go to the road constructions/repairs, I'd rather have better roads than save a buck. Too bad they can not seem to get the roads up to par with all that they take in as revenue.
CD