Disclaimer: These things do vary from state to state. That being said....
I was told just the other day that the police here are being much more strict about roading unlicensed equipment than they were a few years ago. And it's not the police who are interestd in which fuel is in your tank, it's the Department of Revenue.
I've heard of them stopping someone trailering his excavator over the road and keeping them their for an hour while they tested the fuel not only in the truck, but in the machine as well. Why they'd need to test the machine, I have no idea. I've also heard of them visiting someone's yard and testing every diesel powered truck that was there.
I was also told years ago, before they even started to dye the fuel, that if you had a license plate on a machine, you'd better be able to document that you'd been putting taxable fuel in it, not fuel that had not had the tax paid on it when you purchased it. By "document", I mean that you'd need receipts that matched the amount of fuel the machine would have burned for the number of hours on the meter.
Fortunately, I've never run into any of these situations myself, but the people who told me about them, I consider credible.
I'd try calling a different DMV office, you can get different answers from different clerks, the one you spoke with may not be real familiar with the process
Isn't that wonderful? Keep asking until you get the answer you want to hear. Wouldn't it be nice if you could know with 100% confidence that when you asked a question of your government that you'd get the correct answer?