We used our Kroger card at Fred Meyer grocery store gas pumps in Idaho, Oregon, Washington, and Alaska on our trip last summer to get 10 cents/gallon discounts on regular gas. So, if you get the card-lock card for use in Canada and already have a Kroger card that you use in the US, you can get good discounts on either regular gas or diesel on your Alaska trip (at least in the towns that have a card-lock or a Fred Meyer).
However, you will find that having a credit card with a microchip, as well as a magnetic strip, will definitely speed use of self-service gas pumps at Canadian gas stations. We got one from USAA before our Alaska trip and were glad we did.
In addition, we get 1% back on all our purchases, not just on gas, with our USAA credit card. So, we were able to get 1% back on our entire Alaska trip--gas, food (including in Canada, where groceries are more expensive than in the US), RV parks, glacier cruises, etc., etc.
Whether 5% back on fuel costs or 1% back on all your purchases is the better deal depends on how much you spend on non-fuel purchases. For example, restaurant meals are often outrageously expensive on the Alaska and Cassiar highways. Of course, we also paid nearly $8/gallon for regular gas at Muncho Lake, so you have to decide which is best for you.
Whatever you decide, RVing in Alaska and Canada is the adventure of a lifetime, so don't let high fuel costs worry you so much you can't enjoy it!