Curbside outlet not working on shoreline- work w/ generator

Does the neighbor have a 50A rig and is it all working?

The bad leg changing sides when using his post sounds more like the campground lost one leg, which happens to be on opposite pins between yours and the neighbor’s post.

Also, the 30A is probably on the same leg as the bad 50A one, at least in your site. Try the 30A on neighbor’s site or even the 20A in yours.