Reading through the comments, Yes there is at least one loop, possible more, that goes from the engine to the heater cores up front and back. At least in Spartan's case.
Along the way Newmar taps into this loop at least six places according to Spartan tech (one of his projects was to determine how to stop leaks at various points in this Loop).
The DEF tank has a tee, the Oasis has a Tee, the heater cores for the cabin heat has a Tee, and I forget the others.
He had me checking all these areas for leaks to determine where teh coach was losing fluid, that was when I found the leak at the Oasis.
Just happened to catch this guy answering the parts line at Spartan.
The crew at Spartan have been extremely helpful during this process.
Additionally, thank you for the comments on a shutoff valve. I'll talk with Newmar and have one installed, when they repair this.
