I have done something similar that "might" work or at least give you something to build on.
Basically install a drip edge along the top area of the frame to break free the water tension and let the drips happen in a different area. I have seen this on some entry doors, they even sell a door trim piece to do that. I think I have seen it on some Jayco's
In the cargo door situation, odds are high this will need to pull the door unit out of the camper insert the new drip edge, then seal it back up.
Here is how I made mine to deflect water off the bottom of the slide floor.
Start with extra wide house drip edge. This gives me a hemmed white aluminum strip easy.
To cut it, score with a sharp sheetrock knife one side. Then flex back and forth and it will split off clean. Use this score and flex trick on house siding all the time to trim out around windows out of roll stock.
Then create your drip edge configuration for the cargo hole. You might want to leave the 90 deg bend on it to mount to the side of the camper. The curved ends you may need tweak to fit. And this does not need to stick out very far to be in the way, just over the cargo door hinge area
It does work. See here with me pouring water down it.
Now no more water ever gets under the slide.
Hope this helps
Current Sunlines: 2004 T310SR, 2004 T1950, 2004 T2475, 2007 T2499, 2004 T317SR
Prior Sunlines: 2004 T2499 - Fern Blue
2005 Ford F350 Lariat, 6.8L V10 W/ 4.10 rear axle, CC, Short Bed, SRW. Reese HP trunnion bar hitch W/ HP DC
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