I would agree with a loose or cracked fitting on the back of the heater (inside of the cupboard). Take some tissue or toilet paper and squeeze it t around each fitting until you find the damp or leaking one. I found that the toilet paper, just a couple of squares will show the moisture much quicker than a visual or finger check.
If all these areas are dry and the water is coming from the insulation around the tank, you may have a much bigger problem.
Ken
2006 Winnebago Outlook 29B E-450.
2012 Honda CR-V AWD
Blue Ox Aventa LX tow bar and Brake Buddy Vantage.