It's not that simple
Read the article "holding tank odor--odor out the vent" in the HM forum reference library for an understanding of what causes odor, how to prevent it, and the answers to your questions about venting. But--as with just about everything--it's always easier to do something right the first time than it is to undo what's been done and do it over...so correcting the vent problem on an existing tank installation is rarely as simple as just rerouting the vent to a new thru-hull. Although ideally, holding tank vent lines should be as close to horizontal as possible, the vent thru-hull can't be located where it's likely to be underwater when the boat is heeled. In fact, keeping water out of the tank is one of the reasons behind running the vents into rail stanchions...another is one less thru-hull = lower cost. But I don't know of any other boat builder who puts 'em in rail stanchions, and other boats don't have problems with tanks filling up through vent thru-hulls. However, the solution to the problem isn't as simple as just re-routing the vent line to a thru-hull. Because the tanks are usually in the cabin under a settee--below the waterline--you'll still be left with a vertical vent. Beneteau seems to have the right idea for holding tank placement (at least the ones I've seen were...they may have been installed by a dealer) on boats that have just a single head at the aft end of the cabin: the tank is in a cockpit settee, behind the head and mounted high enough to allow a very short horizontal vent off the top of the tank to exit the hull right below the toerail. However, that may not be possible in an aft-cabin boat. It does help considerably, though, to re-route the vent line out of the rail stanchion and straighten out the vent hose--get the arch out of it. Adding a second vent line (if you can figure out where to run it) also helps, at least enough to allow Odorlos to work in most tanks. But short of completely redesigning the whole system, there's not much else you can do.
