We traveled a lot during the Winter months and we found that in the front and rear fiberglass end caps, they used fiberglass batt insulation that had sagged. This allowed the interior surfaces to sweat from the cold outside and the moisture inside. We discovered it when it was dripping on to our dash. I removed all cabinets so I could get to the inside of the fiberglass end caps and used two part closed cell spray foam to insulate the inside of the fiberglass. This took care of the problem and made the coach much more comfortable in the Winter, at least down to -5 during one trip.
You might want to check your end caps because Monaco was using fiberglass in 1990 and it could have sagged away just like ours did and allowed a lot of moisture to develop inside the coach.
