How easy it is to replace the thermostat with a generic one depends on what model of air conditioner and thermostat are in place already. Some use proprietary (digital) communications between the thermostat and the rest of the system and ordinary replacements won't work in those cases. Many, however, are pretty much standard and most any battery powered thermostat will work.
I upgraded the thermostat in my RV not long ago.
Here's a post with some details. It was pretty straightforward once I figured out the various wire connections and naming/coding schemes.