Tank rinsers are avl in any rv store.
water heater tank rinserWe use bottle water for drinking and cooking too.
Draining the WH by using the drain plug always leaves an amount of water in the bottom of the tank, this wand mixes fresh clean water to that leftover water . Run it for a few minutes to allow the clean water to dilute the old. Insert it thru the drain port and let it run to flush things out.
I allow the fresh water tank to drain after a trip, then replace with new water when leaving on a trip.
2x a season I add a 1/2 cup of bleach to the FW tank, run it thru and let it sit in the tank and lines overnight, then drain it and flush things out with new water.