I leave the water level low in my fresh water tank after each trip, but do not drain it during the camping season. Before a camping trip, I will bring the level up to 1/3 for anticipated hookup campgrounds (for flushing the toilet and hand washing in transit), or fill it for dry camping.
I do have municipal water, which is slightly chlorinated. I figure that keeps things from getting funky while sitting in the tank. We use our water for all purposes, including drinking.....when beer is not available.
I sanitize the complete system every spring when dewinterizing.
Chap , DW Joy, and Fur Baby Sango
2006 Ford F150 Super Cab 4x4
2008 Surveyor 263