My Montana did the same thing for a couple of years. It was disturbing. Sometimes, the water would drip over the roof and into the gutters and out the gutter down-spout and drip to the ground.
Other times, it would drip from under the trailer. I attempted to follow the water route on the roof and think I finally figure it out.
I noticed that when water was pooling in the very corner of the roof, where the back of the camper and the side of the camper came together, there was a low spot there in the roof. I also noticed, no water was actually coming over the roof. Looking further, I realized there was no sealant on the joint where edge seams came together, right at that corner.
I think what was happening was, the water was pooling in that corner, not enough to drip over the side, and flowing under that trim. The water would then run in the channel trim that runs vertical on the end corner of the camper. That trim runs under the trailer, follows the J-panel (skirting) where the end is open, allow it to drip.
In other words, that corner trim on the end of your camper is working like a down-spout gutter on a house. The entry point for the water is a roof seam (on the edge) that is not sealed.
I ended up purchasing a tube of Alpha Roof Caulking (My roof is an "Alpha" so, even though Dicor will work, I prefer to stay with "Alpha" branded caulking). I ended up pumping about 3/4 of an inch thick of the caulking into both of the roof corners (on the rear, where the roof sloped downward over the bed room (rear bed room) and filled in that bucket, also sealed that exposed joint in the roof trim.
Well, it must have worked, because I never had water dripping under the trailer again. It now cannot pool in either corner as the actual corners are filled with caulking which is actually higher than the sloped roof right now. The water flows around that spot and over the edge into the gutter.
While I was up there doing this, I also got the same corners in the front of the camper too. They did not have this problem, but I did see where the trim joined together, there was no caulking there either. So, I made sure I filled that in also.
And an FYI, I've not had water dripping from the AC under the camper since.
Check the corners of your roof. I but you'll find a similar unsealed trim area also. Of course, when you do the caulking, Dicor, or Alpha, or whatever, make sure the roof where you will be applying the caulking is dry and clean so it seals good.
To plant a garden is to believe in tomorrow.
2019 Montana High Country 375FL
2014 Chevy Silverado Duramax, 6.6L Dually