As far as filling the tank without a hose, some folks use a large water jug with a short hose and set it on the roof to drain into the tank. That way you don't have hold the 5 gallons while it empties.
Sanitizing a water tank is a PITA. So unless you are sure the stream water is disease free I would avoid that as a water source.
A friend of mine told me that on a recent camping trip he filled the water tank in his Airstream trailer by filling a 5-gal container from a stream (several times). I asked him, "What about giardia?" He said, "We don't drink it, just for other uses." and then, "As a kid we drank from streams all the time while hiking." I did the same thing....but I was a kid in an earlier millenium.
I'm sure that the danger of infection from most wildland streams is way overblown...but still, I wouldn't do it. I hear that giardia, while curable, is pretty unpleasant (at a USFS campground in WA, a sign next to the stream warned of "explosive diarrhea"
) -- but I've never experienced it myself. I do know that diarrhea and camping aren't a fun combination. But I don't really know how common a problem this really is -- I'd be interested in finding out. I'm sure it depends on where you are, how close to a clean source (deep spring, fresh snow-bank, etc.) the surface water is, etc. Maybe I'll look into it.