As usual, you all have given me great insight. I think I'll take all of your thoughts and add it to what I learned from one boat builder's website
and take this a step at a time:
First, I'll go ahead and extend my vent. My heads are direct-drop, so my tank gulps a fair amount of oxygen with each flush. I don't smell anything with flushing; the only problem is with the vent on top. So if I move the problem up to where the lighter-than-air smelly gasses escape upward, I won't care if I have a little anaerobic activity in my tank. I was going to use white PVC, but now I'm thinking that black ABS might aid in heating air and causing a draw much like a fireplace flue. This may be as far as I have to go.
Second, if I have any remaining odor, I think I'll add a 1.5" threaded fitting to the top of my tank at the opposite end from my existing vent. I have room to reach in, drill a hole, and solvent weld a fitting in place. I also have room to add another vent that rises above the tank. I could even add an air admittance valve like this one
. Either way, the tank should then take in lots of air to replace that which rises out my newly-extended vent stack. If it works, it will be much like a Groco Sweetank system, but it will be solar powered. I can brag about this to my green-minded friends.
Third, if I still have odors, I can take the action Peggy recommended. It turns out that 1.5" is the exact size a Groco Sweetank needs to screw into the top of my tank. So the Groco bubbler would just go in place of my 1.5" second vent. That would be the gold standard, but I'm betting I won't need to go that far.
I hope to report back with good results. If anyone has any criticism of this plan, please let me know before I do anything I'll regret.