If there is a vent fan in the ceiling in the area, pull the interior parts and look for signs of a leak. Water could be running down the back side of the interior sheathing. Check the roof caulking in the area, and along the top of the widows and door. I haven't had the problem, but that's where I'd start. Other ideas are the front windows, and the belt molding. Do you have panoramic windows? They have been a source of problems at the bottom corners.
“You cannot reason someone out of a position they have not been reasoned into"
Al, K5TAN and Missy, N4RGO WBCCI 1322
2002 Classic 30 Slideout -S/OS #004
2013 Dodge 2500 Laramie 4x4 Megacab Cummins