Most RVs are pretty tight. One of the things we first learned was to not have any of the vent fans running when using the toilet. They will pull odors all through the RV, especially the one in the bathroom.
It would seem a small fan installed on top of the vent pipe would be the logical solution to black tank odors.
Shouldn't be too hard to put a water tight fan similar to a computer fan or something similar into a housing over the vent aimed to the rear, of course. Routing the power wire would probably be the small challenge.