Cabover area of Class C's are common source for water issues. Sometimes it's the front facing windows (PIA to fix), sometimes it's just the side windows which can be removed/re-caulked, sometimes it's the running lights, roof antennae etc.
Finding the leak source is priority 1. If me I would spend $100 or so and have the rig pressure tested - should identify all the leak sources and save you much time. In the interim I suggest you park your rig so the front is at least a couple inches higher than the back - this will encourage rain to shed away from your likely leak source - might help until you find/fix the problem.
In my case the leak source was a side window - I watched a youtube video on how to fix and it was 1 hour easy fix. So easy I did the same for all the windows when I had some free time.
Water leaks in the cabover area will migrate to the lowest level - that's often the front nose of the cab area - depending on the rig you might be able to drain that area by removing some trim which runs on the bottom of the cabover above/front of the front chassis window. Regardless - make sure you don't have standing water or wood rot your cab area .... zillion posts on this type of issue.