To keep your water from freezing, either you'll have to put it INSIDE the bus to keep it warm enough, or if you mount it underneath, you'll have to have a heated compartment for days when it gets below freezing, or a heating element inside the tank itself to keep the water temperature up above freezing.
Someone might have other suggestions, but that's all I can think of off the top of my head.
Lots of people put them inside, underneath a raised platform for their bed, even when they aren't fighting the frigid below freezing temperatures of parts of Alaska. Keep in mind, though, southern parts of Alaska don't get nearly as cold for nearly as long so you may not have to worry as much about it if you're in, say, Juno or Anchorage.
You'll have to worry, but not nearly as long as if you were somewhere further north. Also, you'll want to consider a bus you can drive over rough terrain as there are plenty of places out there that are barely better than ruts in the ground leading in the generally right direction. But again, that depends on what you're going to do with it.