Properly functioning waste systems shouldn’t smell or need filters.
Key items are proper treatment and good airflow. Many add-on filters actually increase the odor in the tank by restricting the vent. It’s a band-aid at best.
Next time you arrive, slowly open the waste pump out deck fitting, listen for air escaping.
If so, your vent line is restricted and the blockage or restriction needs to be fixed.
Pump and rinse the tank a few times and then add your choice of tank traeatment. I found the cheap RV stuff from Walmart works just as well as the overpriced marine version.
Trace the vent line and be sure that it’s clear. If you find a filter, be sure to check it it’s flowing. Personally, I’d remove it. Many times a blockage occurs when the tank is overfilled and the waste gets caught in a low spot or clogs a filter. Blowing air thru the line will help insure you have a functioning vent.