Gunfire? Eww! Move to a different country.

If you don't want to do this for some strange reason of your own, what about asking the owner of the trailer park to let you move to a different site? Maybe "I really want an east-facing site, for luckier feng shui" rather than "I don't want to live next door to people who kill each other" - after all, he has to rent the site again ;)

If you can't move, then don't bother calling the cops, call the drug squad or the anti-illegal-guns squad (if there's such a thing) directly - assuming that you think they're trustworthy, and your neighbours won't find out who grassed them up.

Play classical music - for some reason this seems to have a repellent effect on young tearaways.

Become invisible to these people.

But really, option 1 - move country - is the sensible one. (Where are you, by the way?)