You can over inflate your tires for a bit better gas mileage, of course you know what will happen to the tread wear if you do. i'd really spend the money on fluid changes, maybe some new suspension part if they haven't been replaced in a while. anything that you think might fail before spending money on off road parts. you could try putting in better gas too for better mileage, my uncle says he gets better mileage in his maxima when he drives from Toronto to Florida. but you should make sure it's ok to put higher octane in without damaging the engine.
should be a great road trip. you could always buy a bunch of stuff for the trip from Walmart and return it when you get to alaska, i'm sure there's a walmart there.... i do it all the time, they never ask why you're returning it...great for when you need tools to fix something on the road, etc.