Got gas at the Vansville bar last year....pretty sure they are cash-only too.
Just after crossing the bridge into the peninsula, if you turn right/east there is a Marathon right on the trail. This was the last spot on the trail where we saw reasonably priced gas, pretty much in line with other stations to the south. All of the stations further north were (admittedly) higher priced.
The next time we head north we'll max them out at the Marathon and spend a little less farther north.....don't remember how much more it was per gallon but I'm thinking .75-$1.00 higher at Vansville and Copper Harbor.