When we had the bad smell, it came from the hot water heater. This is what I did. Drain all the water from the hot water heater. Re-install the drain plug.
Then I take my 25 foot water hose, be sure all water is out. Then hold both ends up while I put 1/2 cup of bleach in the hose. Now, make sure the bleach doesn't run out while you hook hose to your city water at the house and the city water supply at the RV. Now run the city water and open the hot water faucet and run until water comes to the sink. Be careful as the water starts to run that it doesn't splash on clothes, carpet, etc. It is a good idea to open each hot water faucets to ensure the hot water lines get the "treatment" also. Now, your hot water heater is full of a bleach solution. Leave the solution in the heater and lines for 24 hours. This will kill the bacteria that is causing the smell. After the 24 hours, run ample amount of water to get the solution out of the tank. Be careful, don't open the hot water heater drain while the solution is in the tank, as the solution will kill grass, or 'bleach' your clothes if it splashes. WFM (Works for Me)