[rheavn]

I don't see a switch, but still check as I suggested.  If they did away with the 12V air circuit it will be harder to point you in the right direction without using a leak down detector on your coach.  A leak down detector will tell you where there are leaks and where there are no leaks. 

Try one other check.  Turn on your 110V air compressor and when it shuts off, shut off the switch to the 110V air compressor.  How long does it take to leak down to zero on your supply gauge?  When it leaks down to zero what is the reading of the two needle brake gauge?

FYI  I looked at the specs on a couple of 2008 Newells that are for sale & they had two 110V air compressors and no 12V air compressor.  Due to the air door and other air uses they may have found the 12V air compressor wouldn't keep up.