Phil,
I did not realize that Montana built a rig without an outside water fill! That just does not seem right.
For what it's worth we had our first rig for 10 years and never cleaned the fresh water tank. And we're still here to talk about it.
We did, however, use a lot of water so that the tank was refilled often. As long as you don't get some contaminated water to begin with you will probably be safe.
Good luck, Dennis