I realize there's no copper in RV's unless it's gas related...I was explaining the only slip type coupling I know about is copper and how it wouldn't apply in this situation. If it's some movable/sliding coupling that doesn't leak or need glue, I'd like to hear more about it

Regardless, that fitting is broken/came apart or there's a pc missing and maybe in that drain pan/box

The connection in the pic doesn't seem to go together but yes, it's usually a pc of pipe inside that large nut extending well down into the top of the trap.
You have to be careful plumbing, you can fall into a trap yourself by thinking you have it all figured out only to leak and visit home depot many more times

Never heard of snap ABS drain fittings before either

It does sound good though, like SharkBite fittings for drains.