I think the best thing you could have done was 'calmly' talking to the mother as to why the boy was in there. Yes, I believe she should have announced herself first and asked if anyone was in there. The mother said "he needs help". You can't judge just by looking or listening to him that he did not have some kind of problem.
If both of you were coloring your hair or drying off, perhaps they didn't realize someone was in there because of no noise and opened the curtain by mistake, but granted, that's no excuse. You could have enlightened her calmly as to why he was in there and what she should have done by first making sure no one was in there. You certainly don't call 911 in this circumstance. Talk to her and if the answer didn't satisfy you then go to the manager. I'd say both parties were in the wrong.
By the way, most RV parks would not appreciate you coloring your hair in their showers. I have seen rules to this effect. It takes a lot of effort to get the color off the walls and floor and buying new shower curtains. Do it at home or in your own RV.