Update: found it!
Not sure how but I think the Ariral ATV refill event was bugged because I used the hook to climb the border fence and found it with about 300 gas and -700 HP near the invisible map border, it was a bitch to get back over the fence but I did manage and got it repaired.
Still no idea why the save menu reset vehicle option didn't work but I got my ATV back so I'm not super concerned.