no power to coleman mach 3 air cond. compressor

Even though I am apparently not qualified, check for 12 volts at the Y terminal at the circuit board. If you have it, bypass the freeze sensor (just short across the terminals). Bad freeze sensor is the most common problem for this.

On strenth of you knowing how to test the items named,I will forgo step by step instructions and say test for signal voltage from t-stat to relay. Most rtu have no stepdown transformer if tempature adjustment/on off controls are overhead on the ac. Wall t-stat controled units generaly have a stepdown transformer. Test for secoundary voltage output if a transformer is present. If there is secoundary output equal to what is printed on coil,problem likly is with t-stat. No or low secoundary volts points to bad transformer of course. If you find postivily that no transformer is present and there is a relay for compressor(highly unlikly),use a jumper to apply 120 volts to the relay coil. Compressor coming on = bad relay. No compressor = bad cool/hi-lo fan switch or t-stat which may be jumper wired to test. If you have farthur questions you must state wherther it's wall t-stat and if a transformer is present. Still no joy? I reccomend having your home and office hvac technician take a look because he is twice as qulifyed as 99% of rv repair shops.