the fresh tank (and the water system) should be sanitized every so often with bleach. if you goggle sanitizing the fresh water tank you should find the procedure. you basically use a dilute bleach solution in the tank and run it through the water system (i don't usually include the water heater), let it set overnight and then drain and flush with fresh water. i do it when before i winterize the camper and again when i de-winterize and maybe once during the camping season.
the grey and black tanks don't really need anything. we do have a built in black tank flush, so we flush it every so often -
HTT: "EscapeII" 2016 Shamrock 23WS (current)
PUP: "Escape" 2010 Rockwood HW 277 (gone)
TV: "Bruce" 2011 Dodge Durango V8 Hemi Crew
Just us gals (me, Sis and our daughters)
We spend alot of money to go sit in the woods