The vent fan left on could be part of the problem, but I would take the first opportunity to thoroughly clean your black tank and see if that doesn’t help.
Chemicals in the tank don’t necessarily eliminate all odor, in my experience, especially if leavings in your black tank have dried and hardened over time.
Someone once suggested to me to dump some liquid Tide and a couple cups of Borax in there, and that did a pretty good job of cleaning things out.
I don’t have problems with my grey tank smelling, but always dump a little PineSol in there when I clean or a little bleach when I am using it for something else.
Find a need and fill it.
🏡 🚐 Cherish and appreciate those you love. This moment could be your last.🌹🐚