[C320-list] Head Holding Tank Vent

[Dave Hupe]

I've had a head odor (not overwhelming, but "there") since we bought our boat that I've tried diligently to get rid of.  A previous owner installed a bigger (22 gallon) holding tank to replace the original 12 gallon one.  I don't know if there was previous leakage that has permeated the underlying structural grid, but I can't get to a lot of the underlying area to try to clean.
I have replaced all of the old waste/sanitary and water supply hoses, anti-siphon valve on the supply line, as well as the tank vent https://www.catalinadirect.com/shop-by-boat/catalina-42/interior/plumbing/tank-vent-fitting-c-320-c-28-c-42-holding-tank/ 
and hose. I do not have a macerator and thruhull. I also installed a new joker valve on the original Jabsco toilet. Otherwise, the toilet seems to be working properly.  I also use freshwater for flushing, except rarely when we use Lake Michigan water offshore. Lastly, I have done frequent pumpouts in order to clean/flush out the system well. 
I noticed when replacing the tank vent hose that the hose was 3/4 inch coming out of the tank, but the vent barb was smaller (1/2 inch ?).  So, the original tank vent hose connection on the upper vent fitting was not tight.  I used a reducer fitting to go from the 3/4 inch to the smaller diameter hose in an attempt to create a tighter fitting on the vent barb.  
I have read where folks replaced their vent with a larger one and supposedly eliminated the odor problem,  Does anyone have that experience and can you suggest a specific vent that will fit easily in the limited space provided behind the medicine cabinet area?
Thanks,
Dave Hupe
1994 C320 (#32) "Mayan Sun"
Holland, MI