Movie with a serial killer who lives in a camper,

So this movie goes like this....
There's 2 or 3 teenage boys and they are trying to find arsenal to shoot up their school prom. They see a random hillbilly looking guy in town so they follow him back to his place in hopes that he has guns they can steal. They wait til he leaves and underneath his camper they find underground tunnels and a missing girl from their school. Would be awesome if someone could tell me the name of the movie. Thanks

Are you sure synopsis goes like that ? I ask because... This remind me of:

Don't Breathe (2016)

Trio of thieves break into the house of a blind man who somewhere in the ''basement'' hold a girl and torture her.

I'm positive. I've tried googling it wording it different ways but can't find the movie. It's driving me nuts lol

In this movie I think the killer actually lives in the camper out in the desert

Can you fill in any more specifics?

For example, I assume this is an American film. English language. A horror or thriller?

But even things like "One of the boys always wore a red jacket" can be helpful.

One thing that keeps popping up on my searches for this is Sightseers (2012).
Don't think it's that though.

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I've seen Sightseers and can confidently say that isn't it. It's a dark comedy/horror about a couple who go on a vacation and there's a lot of death and mayhem. No teenage boys, no kidnapped classmate.

It was the first title my brain suggested when I heard "camper", though!

Seems like an interesting movie but doesn’t really ring a bell