My husband and I traveled the Alaska Highway, in a pickup pulling a 5W trailer, a number of years ago in July. We made few reservations. The only places we actually made reservations were for Anchorage, Fairbanks, and Seward. Any place else along the way, we were always in early enough not to have to worry about it. Also, we agree with Lifey about Ft Nelson: stay on the northwestern end of town, not at the truck stop place. We stayed at the truck stop place on the way up, and then decided that we were not repeating that experience on the way back, and stayed at the RV park.
Donna