Air pressure is gone, stuck in a Monaco Dynasty

The same thing happened to me last week and I have a 1996 Monaco Dynasty. The problem was the air dryer which needed to be replaced. The mechanic took off the old air dryer and put a rag into the area where the dryer was which built up the pressure so his helper could drive it back to the shop. I actually crawled in underneath the RV and watched him do this. They told me that they had to stop once to rebuild the air back again but no problems.