if you're hitting the Arctic watershed in Ontario it will be Hwy 11 through the north. This is the major truck route as it is somewhat flatter than the 17 (Trans Canada) that skirts along Lake Superior for a ways. Both 11 and 17 meet at Thunder Bay.
From Thunder Bay west there are 2 choices, Trans Canada hwy will take you to Winnipeg through Dryden and Kenora in a northwesterly direction through more of the Arctic watershed.
Option 2 is west and sw from Thunder Bay over to Duluth, MN.
Highway 11 through the north should have good services in all the major centres as for fuel I can't really say if they will have high test or not. Check out www.gasbuddy.com
, you may be able to find current pricing for high test up that way by checking places such as Kapuskasing, Cochrane, Longlac and my favorite Beardmore.
All of your travel through Northern Ontario from North Bay either North or West is on the Canadian Shield. Beautiful country but as other have said watch out for deer and moose at dawn and dusk.
The shield ends about 100km east of Winnipeg and you pop out onto baldass prairie that is so flat you can see your dog running away for 3 days.
Keep us posted when you leave and know the route and we can update you along the way.
Your Visa and Mastercard will work in all the gas pumps, there may be places off the beaten track where you can fill up without even inserting a card first, imagine that eh! Your insurance is good too. Don't bring a handgun into Canada and make sure you have a passport to get back into the US.
Good luck and have fun up here!