No one has mentioned Border Collies. I was seriously ill several years ago and she was most helpful to me. She is calm, patient and a great guard dog to boot. I also live in a single wide trailer. I believe they are considered medium size dogs, although she is bigger than most I see. She has moved down the road with my estranged husband but whenever he's not home she's posting guard duty right on my doorstep! I think she'd just as soon stay here if I'd let her LOL