RV drain leaking at swivel tray adapter fitting

Leak is at the red circled area…

Is this something that is commonly fixed by just putting in another flat rubber washer or do I need a whole new fitting?

If I get a new fitting is there a way to replace just that fitting without having to also replace the 90 and clean out?

I have a little pipe left below fitting to work with but drain pipe can’t be flexed up much more….is there a coupling that would give me some extension and allow me to make this work?