This sets up a tunnel from localhost:6432, to rds-instance:5432, via the host at ec2-instance
ssh -N -L 6432:rds-instance:5432 ec2-instance
psql -h localhost -p 6432
You will postgres tools for a version that matches the version of Postgres on the RDS instance.
Basically, if the RDS instance is Postgres 9.5, you will need postgresql-9.5 client tools to use pg_dump.
cd /tmp
wget https://ftp.postgresql.org/pub/source/v9.5.4/postgresql-9.5.4.tar.bz2
tar -jxf postgresql-9.5.4.tar.bz2
cd postgresql-9.5.4
./configure --prefix=$HOME/usr/local/postgresql/9.5.4
make && make install
# delete the source
rm -rf /tmp/postgresql-9.5.4*
Postgresql 9.5.4 tools will now be available at
$HOME/usr/local/postgresql/9.5.4/bin