Several things to check...
However, if there's no odor in that area except when you flush the head, I suspect a partially blocked vent line--especially if you consider the vent to be an overflow and routinely allow waste to flow out of it (which you should NEVER do!). If it's even partially blocked, flushing the head will slightly pressurize the tank, which can force gasses past the fittings, even if they don't actually leak. If your hoses have permeated, you prob'ly have a faint odor in the boat all the time, it's just a lot worse when you flush because waste going through it can put enough pressure on the inside of the hose to "push" odor that's already soaked through the hose out. If the tank has an inspection port, it's also likely that the o-ring or gasket under the cap cover has dried out and needs replacing. So first, you need to make sure your vent line and vent through-hull are completely open (and stop using the vent as an overflow! 'Cuz if you keep doing that, sooner or later you're gonna have a complete blockage and that will cause REAL problems! Nothing should ever pass through the tank vent but air). I'd also test the hoses for permeation. There are articles in the Head Mistress forum Reference Library (on the HM forum home page) that explain how to do both. I suggest you also read all the articles there...because you shouldn't have odor out your vent when you flush either, and the article on Holding Tank Odor (Odor out the vent)--Cause and Cure explains why you do and how to cure it.
