I would replace the rusted out sheetmetal with an aluminum sheet. On my 74 Argosy, Airstream used aluminum sheeting for that application. The dashes between 74 and 77 are somewhat different but the idea remains the same.
More than likely you'll find that the sheet metal is fastened along the edge of the windshield rubber with screws, rivets and vulkem. At least that's how it was on my 74.
You can see my efforts in dealing with under dash issues in this thread.
There are only two ways that I can think of for that sheet metal to get so rusted, either your windshield seal leaks really bad or there has been continuous heavy condensation on the inside of the motorhome. Being that you're in the Gulf area condensation wouldn't surprise me a bit.