this is a typical concern or issue, or aggravation, for most coach owners - most have found that the easiest way to handle it is to 'level' your coach with that particular side/end a tad bit higher than the rest of the coach - not so noticeable on the inside, but enough that the water wants to run somewhere else!
a secondary measure is to attach sometime to divert it, but these are sometimes a 'guess' as to really where to mount it, as water sometimes takes different paths than we think, sometimes easily bypassing our best intentions - I've used a 'double u' seal made for windows. It has adhesive on the back, and you can easily cut it to the length you need. I mounted ours on the edge of the roof line, right above the passenger side front window, where the water 'tends' to drip off the roof from the front AC - and it's annoying because that window is a side-to-side slide window - so if the water is dripping down it, you can't keep it open, which we like to do. The length of seal was only about 2' or so.
I eventually removed it, though, when I realized that a slight 'leveling' change in the coach could easily remedy the aggravation.
Some would even say that you could install a drip 'hose' from the AC unit itself, to 'where' you wanted the water to drip. That's a great idea, if you can find out how to attach it, and how to keep it from moving while traveling.
two Campers, two Electric cars
former diesel pusher traveler