AWS Postgres RDS dumping data into sql format
There is an interesting thing in RDS postgresql.  I had the same issue, normal pg_dump won't work. Use the below command

pg_dump -Fc -v -h [endpoint of instance] -U [master username] [database] > [database].dump 

Refer link: http://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/PostgreSQL.Procedural.Importing.html