Early this season the hoses in the entire septic system were replaced.
Prior to this the holding tank was flushed 3 times and with a couple of chemicals:
1st flush nothing but 15 gals of City water.
Second, about a quart to 1/3 gallon of bleach and 15 gals water.
3rd, 1 gallon of white Vinegar and 15 gal of city water.
One can smell the chlorine in the city water so that's always there anyway. Needless to say upon pump out and removal of hoses it stunk pretty good. I only put city water in it for the next week to help find leaks whereupon I needed to pump out a fourth time and toy around with the connection to the head outlet.
Since then there has been a regimen of Odorlos and a moratorium on solids going into the head.
No paper, no solids, only a dose of Odorlos, soapy water from hand-washing, city water from the freshwater tank and of course the nominal use it was intended for (urine only).
Odorlos has been used per instructions ( well maybe sometimes a little more and sometimes a little less) and the tank has been pumped out 3 times during the season.
Whenever the head is pumped the smell from the holding tank vent pollutes the cockpit and sometimes the salon below.
Filling the sink bowl about half way and flushing in Wet Bowl mode does put quite a bit of air into the tank.
Putting a second hose clamp on the port stanchion connection eliminated the smell inside by the Nav station unless it wafts into an open hatch/port.
I searched the forum and all of the feedback about Odorlos was very positive.
Now the question at hand: Why is Odorlos not working under these conditions??
In your post you mentioned some soapy water from the sink being pumped into the tank. The instructions on the Odorlos bottle specifically says, "do not use with any other product including cleaning products'. The soapy water may be the problem.
Even Odorlos won't work in a tank that's totally anaerobic...so at least part of your problem is your use of gray water (used sink water) to flush....body oils, soap scum etc may be creating an "oil slick" on the surface in the tank. Whether it is or not, it's not doing the rubber parts in your toilet any good.
The rest of your problem is due to inadequate ventilation in the tank...if your vent line runs into a rail stanchion, your tank vent is barely functional at all. Fix that and it should solve the problem. Email me and I'll help you figure out how to fix it.