I don't know if they are still available, but it seems to me that there was once a "drip pan" available to be mounted under roof ac units. They were designed to route the water to one corner of the pan where a hose of some sort could be attached to further direct the water to any drain location desired by the owner. I have no idea of the manufacturer or where they can be obtained, so I'm not much help here.
Our last coach had two roof ac units. We had the same problem with water draining from the roof that everyone else had. In our case I simply didn't worry about it. Kept the roof clean and did not have a problem with black streaks. Our current coach has basement air and the drainage is no longer a problem.
While I have seen the spouts mentioned above, I don't believe I have ever seen "clip on" spouts. It would be a problem keeping "clip on" spouts attached while traveling our highways.
Tom and Barb
'07 Winnebago Voyage 35L
Workhorse W22 chassis FMCA 219315