On Airxcel air conditioners, there is a low temperature probe that is placed in the evaporator coil. If you open the filter housing and shine a flashlight up into the air conditioner, you'll see a "probe" that is stuck into the evaporator fins. The purpose of that probe is to prevent evaporator freeze-up conditions. If it is not in good contact, is loose or partially pulled out of the evaporator fins it will not function properly and the A/C will freeze up.
There are a number of other issues that could also cause the A/C to freeze, but the first thing I'd check is to make sure that probe hasn't been pulled out, vibrated out or was never installed into the evaporator coil.
You can find some information on it at:
http://www.rvcomfort.com/rvp/pdf_documents/1976307.pdf
Here's the "gist" of the explanation:
"All air conditioning equipment is subject to freeze up when
evaporator air flow is sufficiently reduced. Ducting of any
length creates potential for reduced evaporator air flow and
system freeze-up. To protect both the installer and Airxcel,
Inc. from conditions that promote reduced air flow and system
freeze-up, Airxcel, Inc. has equipped the ceiling plenum
compressor control circuit with a low temperature probe. The
low temperature probe monitors the temperature of the air
conditioner evaporator coil. When the temperature of the
evaporator coil drops below 28 degrees F, the switch will
open, stopping compressor operation. Compressor operation
will resume once the evaporator warms to 55 degrees F."