Why is pg_dumpall throwing an "OID does not exist'' error?
do:

select * from pg_class where relnamespace = 7549789;


and:

select * from pg_proc where pronamespace = 7549789;


These are the most likely suspects (there are other objects that can be related to schema).

Make sure you run the queries in dealermade database.

After you will see the results - answer can be obvious (for example: delete from ... where ..), or more tricky - depending on what is the bad object. But at least you will have some data.

Also - try enabling logging of all queries, and rerun pg_dump to see what was the exact query that failed.