I, too, have a gravity fill.
However, I have found it is much easier to put the bleach in the hose, put the hose in the gravity fill hole, hook it to the water source, and turn the water on. Besides, that way I don't spill bleach down the side of the coach!
As for "wasting" the water by draining and flushing the tank, we use the water for cooking and drinking as well as flushing the toilet. We don't want the high concentration of bleach in our drinking water, so I drain and flush. IMO, the water isn't wasted. Most of it seeps through the ground, getting purified as it goes, and ends up in the well (just like the effluent from the septic system drainfield does). The rest evaporates and comes back as rain.
