"Valve is in discharge overboard"
where are you located? you do realize that this is illegal in US waters and up to 3 miles offshore, rigth? if you get borded and they find your valve open, or even closed but NOT LOCKED, you will get a fine.
If the odor is outside the boat, it's coming out of the vent. besides tank treatment, your best bet woudl be to add a sealand charcoal filter on the vent line. about $80 or so.