Now here is something I was forced to invent. My new to me camper has a force feed to the fresh water tank. Not a gravity one. I bought a standard filter casing that would normally go to a washer. Small bucks. I also bought five feet of garden hose. When I want to sanitize in the spring I will fill that filter casing with bleach. Connect it to the fill port with the hose I bought. Purge the water hose and filter first. And then connect the same to the casing filled with bleach. When the water goes into the tank it will have enough bleach to kill the critters.
All ways keep the fresh water tank full in the summer in my opinion. No air = no critters in the water.
The hot water heater is a simple fix. Drain it from the outside. Squirt bleach into the drain hole several times with a kitchen baster. Screw the plug into the hole and turn on the pump to fill the hot water tank. Let sit for a day and then just use it till the chlorine smell goes away. Showers will be chlorine fresh for several days.