Vented loop won't help your problem...
'cuz odor out the tank vent line is coming from inside the tank...nothing you do to the toilet or the plumbing will affect it. Catalina runs tank vent lines into rail stanchions, then cuts the smallest possible opening in the stanchion, a system that's guaranteed to produce odor out the vent line because it's guaranteed to turn your holding tank into a anaerobic septic tank (for a complete explanation of what's going on in the tank and why that's bad, read the article "Odor Control in Holding Tanks" in the Head Mistress forum Reference LIbrary). Every time you flush, the new "incoming" displaces air in the tank...the only place it has to go is out the vent line, and if that air stinks, you're gonna smell it every time you flush the toilet. There are a couple of possible solutions: reroute the vent line to a new thru-hull, taking as many bends out of it as possible. Then try a product in your holding tank called Odorlos--available from West Marine. That may not cure it completely, but it WILL help a lot!
