I have a 79 31' with the Armstrong 13.5 btu A/C with heat strip. I'm very happy with the job that it does in cooling. With three awnings out and the rock guard down, the trailer can be kept below eighty degrees when it's over a hundred outside. I imagine it could get it down to seventy if I wanted to.
The heat strip can keep it in the mid-seventies inside down into the mid-fourties. In really cold weather (such as it gets in Deep Dixie), I run the heat strip with the furnace set to come on at sixty-five. A couple of weeks ago, it was thirty-one degrees here. When I got up, it was seventy-two degrees in the back bedrom according to the digital wall thermostat I put in for the furnace (a Hunter unit for less than $20). I went into the kitchen, got on my knees, and felt the heat duct. If the furnace had run at all during the night, it was already cooled by the time I got up.
For about a week since then, I've left the thermostat in the same place for the A/C, switching it to cooling about 11 AM and back to heat about 7 to 8 PM. I like to keep it around 78. Right now it's 64 outside, 75 in the front of the trailer, 78 in the bedroom, and I have not switched to cooling yet today. It's supposed to hit the seventies by this afternoon, but we have cloud-cover, so I think I'll just be opening some windows.