Window Replacement - looking for source or advice

Buying a old motorhome, especially an orphan where the maker is out of the business will require some ingenuity to replace or repair odd items like non-standard tail light lenses. I own a 2004 Fleetwood Tioga 26Q, I did get electrical schematics that were generic and not very helpful in troubleshooting. Generally the factory customer contact will advise owner to refer any repairs , etc., to their dealer's service department. I imagine that this is typical with most volume-selling RV lines.