Not sure about the '74 but on most of the 80's + Newells there are several spit valves. There should be three air tanks that are cylindrical, a front brake tank, a rear brake tank and a supply tank. Each should have a petcock on the bottom of the tank to drain out the water. There should also be a dryer near the engine. At least in the 80's+ Newells the dryer has an automatic blow-off valve so when the engine air compressor reaches max pressure (typically 125 psi) it makes a whoosing sound and blows the moisture removed in the dryer automatically. The dryers on the classic Newell's I have seen were mounted on the front wall of the engine bay on the drivers side and say BrakeMaster on them.
Hopefully someone with a 70's model can chime in.
Be sure you have the coach on VERY strong jack stands or cribbing before opening those valves. Dropping a Newell on your chest can ruin your entire day.
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