I had a similar problem several years ago with my 88 camper. I
discovered that the drain tubes were plugged.  If you remove the
exterior rear air intake vents (the large vents between the left and
right rear windows and the rear hatch) you can reach inside and pull out
the drain tube.  Pull off the caps at the end of each tube.  The caps
have a pin-hole that gets clogged up. I replaced the caps with some
nylon netting that I secured to the end of the tubes with a plastic
cable tie.  I have not had a problem since. 

 That is with the draining, the A/C has always been a constant headache.
 I find that I always have to have it recharged every season without
anyone ever finding a leak :<(

Tim Fast
fastth@phibred.com

88 Westy (The Battle Bus)