I don't know of any campground that does not require pets to be leashed. Why then, do people NOT leash their dogs? If you can control your dog with one word, no problem, but I'm not talking about thos...
I'm super vigilant on walks, looking ahead for loose dogs. If I see a loose dog, I turn immediately and walk the opposite direction, yelling at the top of my lungs, "Please come get your dog - NOW!" Fortunately, I don't remember the last time I've had this problem. If the owner isn't cooperative, I'd have no problem in reporting him/her to management.