Mike, Very interesting conversation. If the coach came from the factory with roof air conditioning, then it would have the drain hoses incorperated into the roof structure which would be visable if you remove the plastic plenum cover right under the air conditioner. There is a "T" in the hose going to each side that goes thru the mounting gasket and up to the cups mounted to the bottom of the condensate catch tray of the evaperator. Over the years, dust,dirt and lint will accumulate in these cups and block the water flow into the drain hoses. If you blow back thru the hoses, sometimes you get lucky and dis-lodge the blockage but the dirt is still trapped in the cup. Another issue is sometimes the drain hoses droop down under the air conditioner and cause a p-trap effect which will block drainage. There is also a negative pressure on the drain system when the air conditioner is running created up in the evaporator compatment. That is evident when you turn off the a/c and you see an excess amount of condensate water draining from the coach.
Hope this helps, Fred
