We did our Alaska trip last year from Wichita, KS. Went south west to Santa Fe to drive the Rockies from where they start in NM till you drive out of them when you hit the Yukon. 10 week trip of 10,366 miles in the RV and another 1,994 miles on the toad. We drove 3,000 miles to get to Mile 0 of the Alaska highway and then only 1200 more miles to get to Alaska.
I notice a 0.5 to 1 mpg hit while towing (see rig in sig. below). The trip you are taking is not about gas mileage, it is about the journey. Take the toad. You will want it for the side trips and it is cheap insurance for anything unexpected that goes wrong.
For the RV we purchased 1547.6 gallons of mid grade fuel to avoid pre-ignition knock while towing in the mountains. Average fuel price (all converted from liters and Canadian $) was $4.43/gal. Total fuel bill for RV was $6,857.89. And our RV gas consumption was 6.7 mpg.
Our trip was July 1 thru mid September. Gas is cheaper now, but who knows about next summer. We had a great trip and I trust you will as well. Enjoy the journey.