[Tiara Sovran] Holding tank smell

[Uhehner]

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