Most truck drivers in the US take a physical and get a two year medical card. Diabetics that demonstrate blood glucose control get one year.
At my last physical my blood glucose was way out of control (my bad), and they gave me a three month card and "come back way better than this in three months."
I then ate mostly meat, cheese, plain greek yogurt, and protein bars (17g carbs) and protein shakes (5g). Pre-sliced apples and frozen vegetables. Minimal cheating, confessed in my diary. Most of my shopping is Walmart, because you can park a truck up in there.
In regular consultation with a nutritionist in my doctor's office (I send her my weekly carb and blood glucose diary), I generally eat anywhere from 40 to 80 grams of carbs, according to labels, or Wolfram Alpha when needed. Occasionally low hundreds.
https://www.wolframalpha.com/input/?i=nutrition
Not quite keto, but low and mostly disciplined. Minimal exercise.
My glucose readings went from high 300s (like I said, out of control) to 90s and low hundreds. My nutritionist saw a few low 80s in my diary and warned me that 80 is moving into "low blood glucose."
Aaannd ... I've lost 30 pounds; 220 to 190, at 5'11". That was an absolute and unanticipated side effect.
The other main diabetic measure is A1C, which requires a blood draw. Last recent physical was 14, double what's reasonable. It's a 3 month average, so it won't be where I want it to be after 3 months, but I expect enough improvement to regain my one year medical card.
https://en.m.wikipedia.org/wiki/Glycated_hemoglobin
All this (and the "minimal exercise" comment) to say "low carbs good, added sugar and processed grain really bad."
As a trucker, almost everything I eat comes in a package, and is cooked in my microwave or eaten directly out of my little refrigerator. I read the labels, and put it back if there's added sugar, or enough carbs that that one meal would dominate my carb budget. I eat oatmeal (with water, not milk, my preference) almost every morning, steel cut and out of the round cardboard container, not flavored floor sweepings in envelopes.
Read the labels. And if you're diabetic, stick your fingers every day, even if you don't use insulin. If you don't read your blood glucose regularly, you could be lying to yourself.
Anecdote of one.
You're right on regular monitoring even without taking in carbs, depending on your metabolism, you can still get crazy high readings on almost no carbs in the diet... Took me 6 months of no carbs and had to still take insulin on occassion to keep it controlled.
Wolfram Alpha tells me a quarter pounder is 42g carbs, and it also tells me that the bun is 23g. So the quarter pounder that I just ate goes in my diary as 19g carbs, and the bun goes in the trash.