[rheavn]

"There has to be a check valve between the 12VDC supply air and the line tying it to the overall supply."

The only check valve I have found, so far, is at the "T" coming out of the 3 gallon air tank where the 1/4" air line from one of the "T"s connect.  That is why I believe this "T" supplies the 1/4" air line that splices into the 110 psi air line that comes from the rear of the coach and then is stubbed off.  That way supply air does not go back into the 3 gallon air tank.  Does that make sense???

"Can you sketch a diagram?"

Yes.  I'll post later.

So far I have the toilets, pocket doors and a 3/8" line that disappears up into the floor that are supplied with 60 lbs regulated air from the 12V air pump.  The slide seal regulator and the dump valves, left & right, are supplied with 110 psi unregulated air.  Note that my coach only has one slide seal regulator that feeds air to both slide seals.  Also on my coach the 12V air pump has a pressure switch just like the 110V compressor. "No two Newells are the same"

Clear as mud ey??????? I'll buy you an expresso at Ruidoso