Do yourself a favor and get the PenFed Visa Cash Rewards card which gives 5% cash back every month on fuel, has no Canadian transaction fees, and can be swiped at all pumps and must be swiped at the pump to get cash back. This is now the only card we use for car/RV fuel. It does require that you open a $25 money market account or pay a $25 yearly fee. Guess which option I chose. Cosco and Arco don’t accept Visa.
Do not fall for the Canadian 3 cents per liter discount at some station as you pay inside. Sure you'll save 12 cents/gallon while I save 25 cents/gallon with my PenFed card.
The following graph should give you a good idea of what to expect for the Alaskan trip fuel prices.
My 2013 Alaska trip fuel graph