You are treating the symptoms not the problems. Polyethylene holding tanks do not absorb odor. Sanitation hoses, especially the holding tank discharge hose because sewage sits in the hose. Over time these hose absorb and reemit the odor. If you want to reduce the odor from the system the hoses need to be replaced about every 10 years. The hoses produced today are much better at resisting permeation and are pretty odor free. That's step 1.
Step 2 is to properly treat the waste going into the tank to reduce odor. There are 2 products that will help Zaal Noflex Digester and Camco TST. The cause of the odor is a proliferation of anaerobic bacteria.
Step 3 is to increase the tank vent size, it is probably a ⅝" hose, increase the size to 1"
These are the tried and true ways of reducing holding tank odor. Get
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Get Rid of Boat Odors
There are 2 main causes of odor in the head, human activity and sea water. There's not much that can done about the first. The second, sea water can't be dealt with. Sea water is filled with all kinds of little critters, plankton, protozoa, and who knows what else that slips by the filter. After flushing this stuff stays in the bowl, dies, and rots. If the boat is used daily the odor doesn't build up because the dead critters are flushed out before they smell. However, most use the boats on weekends and over the course of the week the critters die and rot. The solution is a fresh water flush, either all the time or the last time it is used before leaving the boat.
A properly charged and maintained battery has minimal out gassing, meaning not really enough to worry about. The gases are Hydrogen and Hydrogen sulfide (the rotten egg smell). So long as the battery compartment is not air tight, the hydrogen will find its way out. Hydrogen sulfide is a bit more of an issue, it can condense on electrical parts and cause corrosion, it is not explosive. The cause is a bad charger. The Charles charger you have is a bad charger, it only has 2 stages, bulk and float. Worse, the bulk stage is timed and not dependent on the batteries condition, this will lead to over charging the battery causing both hydrogen and hydrogen sulfide to form. Over time this shortens the battery's life which leads to over charging. Install a modern 3 stage charger from Victron or Sterling/ProMariner and new batteries and you will not have to worry. It is certainly completely unnecessary to add a noisy 3" inch inline fan to ventilate the battery compartment.