Remove and reseal the window, could also be the slide seals or any seams including the roof area. Finding the source can be difficult, even not possible as water can travel a long way. Check and reseal everything including everything it can't be like a roof AC or other windows.
But sounds like it's the slide. Prior rig had slide roof fasteners that weren't caulked.
2009 Holiday Rambler 42' Scepter with ISL 400 Cummins
750 Watts Solar Morningstar MPPT 60 Controller
2014 Grand Cherokee Overland
Bob