Advice from experienced RV trqvelers in Canada/YT/Alaska

We are also planning a similar trip. We have pulled our Spree through the lower 48 with our 1500 Suburban and not had difficulty towing up or down, but concerned about the highway grades at Fort Nelson, Watson Lake has anyone done this before. Milepost list one of the as 8 to 10 percent! Also we cannot leave CA until the 1st of July. sand plan to return mid september When should we be concerned over the weather changing for the route home. Thank you for any/all help!