Dark animated drama about dogs, surviving reservoir. Similar style as watership down, only dogs. Style hints to a 70s/80s era.

Sounds a lot like "All Dogs Go To Heaven" from 1989. Burt Reynolds voiced the main character.

Try The Plague Dogs

I know they made some dark animated films in the 70s and 80s but The Plague Dogs is pretty shocking. I bet kids found that one really terrifying.