Am a novice myself but here goes.
In park the engine hasnt got the friction of the transmission, only the weight of the flywheel etc. So when you put it in reverse or drive for that matter the revs drop due to that friction and you might mistake the issue being the auto when it is more likely to be an engine running problem.
I'd be getting my engine running nicely first as a priority.
Idle revs should be around 900rpm.
www.youtube.com/watch?v=HXwh4z8d66o
SUBARU ea81, VW auto, Detour camper
My ex girlfriend said "its me or a trike". Her big mistake.