I've been on the boat for four days since we finished upsizing and relocating the vent. No more odor! I've used Noflex in the past, and would use it again, but Hopkins Carter was out of it the last time I was in, so I've been using KO instead. Honestly, neither of them worked for very long before the bigger vent, but I bet they both work now.
As far as the fittings go, I probably could have looked harder and found a 1 1/4" thru hull, but I needed to get the job done and west marine had the parts to do it in the 1 1/8, so that's what I did. FWIW, I don't see much point in going with a hose and through hull any bigger than the threaded opening in your tank. The size of the opening will constrict the air flow, even if it is attached to bigger hose, correct?
Either way, 1 1/8 seems plenty big for a vent hose. Way more airflow than the 5/8 that was on there before.
I may use one of Peggie's cool seals and reattach the old vent as well, even though it's small, it would help a little, and it is already run.