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.
