It is probably not the microswitch. The switch in question might be a low pressure switch, and that shuts off the compressor when the freon pressure in the line is less than about 10 - 18 PSI. This happens when the freon coil is cold, or due to lack of freon in the system.
Anyway if it was a relay, they should not be more than about $10 - $20. You can probably get one from a pickup in the junkyard for $1 or so.
It would help to know the model year, chassis manufacture, and more details about the RV to help solve this problem.
Have you had to add freon in the past? How long ago? What year is the RV? What year did you buy it?
I know a couple of people who run their generator and rooftop air instead of fixing their R-12 freon dash air conditioner. It is just to expensive in some cases.
Fred.
