Wintertime in cold climates is typically very low humidity. Condensation in an RV in that climate is caused by the sun heating the interior during the day and the nighttime cooling causing the RV to 'inhale' slightly more humid daytime air.
If there is no open vents or windows there is no way for that moisture to get back out and it just builds up and up.
As was stated, merely leaving vents or windows open will prevent this.
In my case I have a MaxxAir vent cover on the bathroom vent, that vent is only ever closed if we're in the unit in cold weather.
In fact leaving it open while driving in cold weather makes the back of the motorhome FAR warmer and less humid too.
Ted 'n' Laurie, plus Jackson (aka Deputy Dog, the Parson Russell Terrier 'fur kid') and, Rylie (who crossed the Rainbow Bridge June 14, 2012).