On another forum, this was discussed and the writer did all the work himself. Turned out to be the air valves but with a specific problem. He replaced the O-rings in the valves but it didn't fix the problem. He then dismantled an air valve and discovered it had an o-ring UNDER the electrical solenoid. Since it was old and cracked he replaced it and that turned out to be his problem. Seeing as your coach is an older one (and mine too) it looks like that's where I would look. I'm hoping to last just a little longer.
Good luck and safe travels!