Just came back from a trip and had the same thing with our main A/C, had condensation coming back in the cold air return vents. I started by cleaning the drip pan as i thought it was plugged and that made no difference. I then pulled the A/C to find moisture coming under the foam seal, i'm assuming my seal has just gone bad. I ordered new ones and am going to replace tonight hope this helps. I think I read a post a while back where someone had the same issue I'm having and their fix was to replace the gaskets as well.
2015- Voltage 3818.
2013- F-350 White Platinum DRW.