If it's any consolation, plead ignorance and tell them you were in the South RIGHT AFTER KATRINA. Practically every diesel pump was dispensing red fuel because that was all there was to be had. The feds gave a special exemption for 45 days, so long as the taxes were collected at the pump, the DOT and troopers were supposed to leave you alone. Yes, they DO inspect our diesel pickup trucks randomly. Just drive down the interstate by a weigh station in Florida, occasionally you will see temporary signs telling motorists driving rental trucks, rental trailers, vans, and pickups to enter the weigh station. As an added bonus, the DOT guys dip the tanks of diesel vehicles at random. Guess who got picked?