Originally Posted by RickDavis
Most tanks have multiple openings high on the side that a 4 inch pipe can go in.. You could also T into the line from the house with a riser to dump in.
Shouldn't need to go that far. I have yet to see a sewer installation that doesn't include at least one cleanout plug between the house and the tank, usually close to the house foundation. Find the cleanout plug, remove the cap, and dump there. Cap it again when you're done. If you're using a macerator pump, you won't be discharging all the time, only when the holding tanks get close to full, so you don't need to keep it permanently connected to the cleanout plug.
If you go with a macerator pump, insert a "Y" fitting upstream of the pump, so you can run both your blackwater and graywater tanks to it. Discharge the black tank first, then the gray tank to rinse the line. Also, use a clear discharge line, so you can see when you're done pumping; macerator pumps overheat easily if you run them dry for too long.