We shaved the price of fuel from ~$6300 to ~$3700 by taking the small Class C in my sig. However, being in a smaller RV had other expenses, like having to go to a out to wash clothes. We also ate out more, but then we might have eaten out just as much in the bus. ;) Part of the fun of going is trying local cuisine.
If you really want to save money, give up on Alaska and go to Banff. ;) The scenery is the most spectacular we saw in the entire trip!