trcgolf,
The Alaskan Highway and the main Alaska roads do not go over mountains, just some rolling hills and a few low passes, basically not high elevations.
So your mileage might be a little less than your local travel. But what you should expect is to pay for costly fuel. In 2013 gasoline was about 20 cents more per gallon than diesel in Canada and that is not a typo.
Our diesel average cost in 2013 was $4.35/gallon and we have a long range and could bypass the $6/gal fuel. If you don't plan ahead for fuel then expect to sometimes pay $6/gal. For 10,000 miles at 7MPH and $4.50/gal the fuel cost is $6,428.
But the trip is an adventure.