I haven't found that Avis AWD codes work in Alaska, due to the specific franchise arrangement there. In Alaskan locations where there is a National Car Rental I have found better rates with them due to an abundance of codes and coupons. Unfortunately, for your specific itinerary it appears that National won't let you do a one way between Fairbanks and Anchorage but perhaps you could use National if splitting up the rentals as suggested by IAHtraveler. Have you thought about driving back to Anchorage? Hope that helps.