The tiny amount of poo in the rinse water would be good for your yard, just let it go in the grass away from traffic patterns. I "rinse" the tanks at the dump station with 5 gallons of water dumped in the hopper and the sink. I dump another 5 gallons in each for the trip home and let it go on the grass when I arrive.
2006 23B Hybrid with 10k round bar WDH
2011 F150 4x4 SCREW Ecoboost, Max Tow, Integrated TBC, 3.73 LS axle, Firestone Ride Rite airbags.