There is no reason to plumb the W.C. (Head) discharge “down” into the holding tank
. It’s not a gravity system, as the W.C. waste discharge is pumped to the tank (or overboard).
The Head discharge is plumbed, through a Vented Loop, to the top of the Holding Tank
. The head is normally lower than the holding tank
“cpttb’s” advice that “gravity is critical”, and “a loop would make problem worse” is incomprehensible, to me
. He’s right that the “line from head leads to top of tank”.
See the basics at “Installing a Head” - by Don Casey