FYI I have a 1981 Dodge motorhome that in around 1997 I discovered the whole frame around the top of the windshield was rotting out. The MH had fiberglass front and rear caps with aluminum in between.
For expediency I opened up enough of the frame to do some work with a rust convertor/preventative and filled it is Cat hair-reinforced Bond. It probably took 2 gallons but it reinforced the who area without welding and is still solid today.
I used it similarly on the rain gutter of my 1997 E-350 after stripping out all the body seam sealer and rust.
My roof paint was bad despite being already repainted once so I coated it is a white elastomeric roof coating. It is tough stuff but it will bleed white and create a film on the windshield.
Bed liner products are probably better and that is what my next project is using.
https://www.amazon.com/Evercoat-868-...a-572414163977
If you are anal and have to weld, go for it but plan on removing the head liner/windshield to avoid burning that and doing much more repainting where you have to burn through to get to the metal.