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
http://www.boatus.com/boattech/casey/13.htm
Regards,
Gord