There is allways an exhange fee, no matter what you buy or if you want money. Major credit cards like Visa and Master card are accepted everywhere. American express is not accepted everywhere and will not be accepted at Costco Canada, they use Capital one / Master card only or cash. Your Amex card will be good to get in, but not to pay.
My tree credit cards charges 2.5% exchange fee when buying something. If I get money from the card, I pay the transaction fee, either a bank or a machine, then I pay 2.5% exhange fee, then interest aplies from the first day on.
I would bring just enough money for the trip and use my credit card anytime I can. You will need cash to use Yukon governement campground and bring some just in case you could not pay for fuel. Campground could be paid in advance, but it is much simpler if you pay at the campsite in the envelope. There is no attendent at those campsite and no host. Also, at some BC governement campsites you have to pay cash in an envelope. At private campground, sometimes you could have a better deal.
This is a good year for your American money, so just enjoy the rebate!