http://www.beamalarm.com/Documents/h...rvice_data.htm
The above ref link has all the info on the typical D2 valve.
As mentioned on other posts, your Newell has 2 aux compressors. I would be surprised if either of the these aux compressors would produce 150 psi. It's possible, but doubtful. If infact you have 150 PSI it is probably being generated by the engine compressor.
Might want to check the gauge as a safety valve would normally prevent that pressure.
As far as safety valves, there are probably numerous ones on your coach. Could be one in the engine compessor, or the wet tank would be my first suggestions.