I don't have a motor home but did find a leak over the rear window in my rockwood. I removed the window and found a leak at the top corner where
they had not used enough putty under the window flange. I know it was leaking there because there was road grime around the under side of the window flange.
Patted myself on the back and went camping.
Next time it rained I saw more drips in the window area
I found that several of my clearance lights had water marks inside
the lens covers! Rusty mounting screws too. I put a tiny bead of silicone sealer around the rim of ALL my clearance light lenses. No more leaks!
Leaking clearance lights are a common problem for RVs of all shapes and sizes. Remember mine weren't leaking where the light mounts to the outside wall. They were leaking at the lens and filling with water and then the water got in thru the screw holes or wire hole.
Easy fix once you get started.
Only tools are a shop rag/paper towel, maybe a small screwdriver and a tube of bathroom sealer. You can use clear but I used white and it's nearly invisible once I pushed the lens back on and wiped off any squish out.
Good luck with your repair.
BTW, I also suggest dry everything do temp patch and take your trip.