I see in here they say both gas and Diesel at all the major stops.
I see you have about the same outfit as we do, truck camper on a Dodge Diesel, my only real worry is flat tires as we are grossed out quite heavy and the trucks tires tend to bulge out on the sidewalls.
I have always found people very helpful anywhere in Newfoundland or Labrador.
Once for work I was up the Northern Peninsula for work in a work van and got a flat. I had no idea where the spare tire was. The grader driver for some small centres stopped and went straight for the 'donut'.
It took me about 5 hours for a service centre to find a tire that fit and to get it changed.
In Labrador there are fewer places to stop. Drive below the speed limit.
We did buy gas, but I am certain that there was diesel at all of our stops. We did boondock several times, including a church on the top of a hill in Red Bay, the Walmart at Labrador City, the gas station at Manic 5, and behind the mall on the coast at Matane after we crossed the St. Lawrence (across the road from McDonalds and by the fake ship). Supposedly there is also a spot in downtown Churchill Falls. There are numerous pull offs along the route. Enjoy the wildlife. We watched a bear for about 10 minutes, and saw several moose. We carried an extra tire, but only had one tire problem and that was caused by dust and was just a slow leak because of the dust between the tire and rim. Easy fix at a station.