Composting heads only partially compost at best. Your last "deposit" is certainly not composted at all. Ventilation is important, as oceangirl's husband correctly identifies the difference between aerobic (needs air) and anaerobic (can't live in air) bacteria.
Anaerobic bacteria are the ones that stink. However, they are the ones that are in your gut too, so you can't fully keep them out. You have to have a good aerobic environment, so they die quickly and are replaced by aerobic bacteria, if you're trying to decompose your waste. This principal is the same in a composter or a holding tank. The biggest problem with holding tanks is that boaters do things that kill aerobic bacteria all the time. Such as, flushing oil, which skims the top of the water and prevents air exchange, or flush chemical cleaners, or don't flush enough clean chase water so waste sits in the unventilated hose, or simply don't have sufficient tank ventilation to begin with. The single biggest mistake is....... flushing chemical deodorants!! They just kill the good bacteria, so once they wear off, the tank is even worse!
A small 12v ventilation fan for a composter makes all the sense in the world to me. Besides the negative pressure, that would draw any odor (even the earthy soil odor from most) to the outside, the fresh air being introduced would create the best composting environment. I suspect, while oceangirl is dehumifying, the real benefit is the constant exchange of fresh air.