I had a similar problem on a 2008 Tiffin Allegro Bus. Every time it rained, I had it, but never long enough to troubleshoot. Tried sealing plugs, etc., with no help. We had no problem with the air system itself, only the gauges and low air buzzer.
The gauges and buzzer get their signals off a single wire that the MMDC sends as a multiplexed signal along the wire. The gauge knows what signal it's looking for and pulls the appropriate data out of the data stream and shows it as a gauge reading. If you look at the back of the dash, you'll see all the gauges are daisey chained, one to the next. I solved my problem by pulling all the wires off the rear of the dash gauges, scrambled all the connections and put them back in the new order. It worked, and I never had a problem again. I don't know why the rain effected the problem, but it never happened again. Go figure???
2013 Tiffin Allegro Bus 43QGP
2011 Chevy Silverado 1500 Toad