Lake Wales, Fla.

In a campground near Lake Kissimmee, Andrew Pollack and I sit in the shadow of a white RV, his spartan home. He broods by my side in cargo shorts and a T-shirt. He’s just sold his large house in Coral Springs, Fla., because he feels “physically sick to be in Broward County,” where his 18-year-old daughter was shot dead last Valentine’s Day at Marjory Stoneman Douglas School in Parkland. He’s lived in his RV for nearly three weeks with his wife and their Belgian Malinois puppy, who isn’t yet at ease with...