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This can happen if the hot water tank has not been flushed for some time.There is a bacteria that thrives in warm, dark, humid places and gives off a rotten egg smell (actually it's sulfur). It's not uncommon when a rig sits a while without being used.
To flush the tank, shut off the city water or water pump. On the bottom of the water heater will be a fairly large drain plug, some water heaters will have a anode rod that pulls out. You can then open the relief valve to speed things up. After it is drained you can turn the water supply back on and flush it out.. After this you can then shut off the hot water heater.
Dilute 1/4 cup of household bleach for each 15 gallons of tank capacity in to a gallon of water.
Add the chlorine/water solution to the water tank. (Never pour straight bleach into the RV fresh water tank. )
One faucet at a time, let the chlorinated water run through them for one or two minutes. You should be able to smell the chlorine. (Make sure you are using the water pump and not an external water supply.)
Top off the RV fresh water tank and let stand for at least three hours over night is better.
Completely drain the system by flushing the faucets for several minutes each. Open the fresh water tank drain valve to speed up emptying the tank.and Open the hot water tank drain plug and drain until it is empty.
Close all valves and faucets and drain plugs.
Fill water tank with fresh water.
Flush each faucet for several minutes each repeating until the tank is again empty. (Make sure you are using the water pump and not an external water supply.)
Fill the tank again. The water should now be safe to drink but if the chlorine odor is too strong you can repeat the fresh water flush.
Also One of the chemicals they put in propane is Mercaptan,this is done to let you know of a possible propane leak, the smell is of rotten eggs. Normally you will have an LP detector and it will pick up on a propane leak and will sounds an alarm and cut the flow of the propane. The best way to find a LP leak is with a LP sniffer or by spraying down the lines with leak check, it will bubble if you have any leaks..
I would first drain the hot water heater and follow the directions above until the smell is gone you should at least be able to notice a difference... IF you have any other questions or concerns please feel free to contact me.