[holiday3900]

I would also check the vent itself. The screen corrodes and prevents the gas from escaping. You can remove the screen with a dental pick. It's only required on the fuel tank lines. But then you still have to check it for insect nests.
I once had a strong urine smell in my boat too. Found out it was a rat living in the bilge ( remember the old saying, you can smell a rat ).
I assume you flush with fresh water. Salt water flushes present additional problems. I always add the green holding tank liquid from West Marine or Walmart after using the head. It seems to work quite well.
Capt. George

Thank you for your reply. We are in Michigan w fresh water. The gauge is working and tank not even close to full. It is Hard to get to the holding tank/ filter but mechanic thinks bilge just needs to be cleaned from non use. Suggested I Pour a few gallons of dawn soapy hot fresh water. Might have done the trick. If not, I'm going to dig into filter next


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