First off lynun, Welcome to the Forum!
After reviewing your Profile I see you have a Georgie Boy Class A so that original overflow tank may or may not have been a Chevy part. Sorry to be ambiguous but pieces like that are sometimes replaced or added by the coachmaker depending on how much of the chassis was modified to fit the coach body.
I think you should be able to use a universal reservoir or as ronspradley suggested, just about any good container with a lid that you can stuff the overflow hose into.
1995 Coachmen Santara 360MB 36' w/slide.
Ford F53/460 chassis, 2020 Chev Equinox "toad"