Just had mine replaced because of torn corner in center of two windshields. Newmar had them in stock for a 7 yr old coach. With older and MFG's not in business makes it hard to find a replacement.
My cost new gasket 113.44, adhesive 115 and labor 750 for total 958, removal clean surrounding fiberglass surface and remount two piece windshield and let cure with heat lamps over night just to give you and idea what the cost maybe.
This
windshield link may help you locate someone that can help for a new gasket if you can not go to your original MFG.
Newmar does repair other MFG's motor homes at their factory, you can call Phil Sillbey 574-773-7791.