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