Could be most anywhere, probably somewhere in the cabover unit. The underside of the cabover (which I think is what you're talking about for where the water is dripping from) is a low point so most any leak in that area will collect and eventually find its way out there...often after sitting around for awhile, getting yucky and causing damage.
Check the seams, clearance lights, any windows in the cabover, etc. for the source. I'd also suggest removing whatever trim and so forth is accessible from the inside of the cabover and seeing how wet stuff is there.