The smell is coming from the bacteria that have formed in your Grey Water Holding Tank for the last couple of years.
You need to dump the tank, but before you do I would suggest that you get some Chlorine Bleach and pour it down one of the drains leading to your Grey Water tank. The ratio should be 1/4 cup of bleach for each 15 gallons of water your grey water tank holds.
Once you add the Bleach I would also suggest you fill up your holding tank to almost full and if possible drive the RV around for about a half hour or 45 minutes so the bleach is agitated in the tank. Then let the RV sit for about 2 or 3 hours and dump the tank. You may have to repeat this process several times to get the tank clean.
Once the tank is clean you should start treating both your grey and black water holding tanks with Holding Tank Deoderant and make sure that you follow the manufacturer's instructions when using these treatments.
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