In that pic, something, maybe a flagpole, sticks out of the ground beyond the RV. At that point in the picture, is the curved image I see something bulging downward? From the bottom of the cabover? I ask because if it's sagging like a pig's belly, there's a lot of deterioration.
My concern is that water's getting inside above that corner, and the loose trim might be doing more to let water OUT than in. 
Can you rig a pressure tester? A furnace blower, big leaf blower, ventilator, really strong fan? Close, maybe tape, all openings, duct the blower or whatever to a window or vent, and pressurize the coach interior. Then spray soapy solution (kid's bubble stuff) around on the outside and look for bubbles. You know you'll see them along that seam, but what you need to find is the source.