The refrigerator Vent Covers are readily available on Amazon. Prices range from $32 - $39. They come with the Frame and the Vent Door cover. If your frame is in good shape then you only need to use the vent door. T
There are two brands - Dometic and Norcold. And they are both the same overall size of 21 1/2" x 13 3/4" - which INCLUDES the frame in that measurement. So the Vent Door itself would be 20" x 12" - (Just what you need).
If you have a Dometic frig in your Class B, then most likely the Vent Covers are Dometic. Or if a Norcold frig, then your covers are Norcolds. Just search "RV REFRIGERATOR VENT COVER" on Amazon and then check through the various choices. There is a smaller size vent cover, so make sure you choose the larger one that is the size you need.
Amazon has a lot of replacement parts for older RV's. I've bought the refrigerator vent covers and roof-top A/C plastic shroud covers from Amazon - as well as tons of other stuff for my older Class B and B+ RV's.
PS I tried to post photos but I don't know what the URL is or where to find it for the photos. Usually you can just double-click to download a photo
Now: 1998 Coachmen Starflyte FRB 21' Class B+
Formerly: 1996 Coachmen M19-RD GMC 19' Class B