you must ALWAYS fill the water heater BEFORE turning it on as the element will instantly burn out if its not full. Also it will heat much faster on gas then electric and if you are doing back to back showers you can use both for more hot water. Your rig may have a black tank flush installed as many KZ's do. You attach a water hose to it turn the water on and it sprays the inside of the tank to rinse out. If not there is a wand that you can get that attaches to the end of the hose and is inserted down into the tank thru the toilet to do the same thing. I always then put a gallon or two of water in the tank along with chemical. This sloshes around and helps prevent a clog from forming. As for the grey water....maybe a rinse that's all. One thing ...dump the black tank...flush black tank....then dump the grey tank last.
