As RDS is a managed DB service for most popular engines, RDS forks the community engine (Postgres in this case) and builds features and automation on top of it. Coming to your query about a place where there is a dedicated difference between the parameters on RDS v/s community engine, I would say there won't be much difference in the parameters. Maybe you will find few parameters which are additional (modifiablle/non-modifiable) on RDS.

What I would recommend is to execute the system variables command on community as well as RDS instance to get a list of variables. Then compare and find what are those few variables which are different between RDS and community.