The most common AC issues are caused by a low voltage source, all too often the result of a voltage drop across too long and/or too small a gauge extension cord, or an overloaded older RV park’s electrical grid.
First thing is to determine the faulty part. If it’s just a start capacitor (there’s two, start and run) that’s lucky and certainly less costly than having to replace that AC plus the rear one; since today’s AC’s don’t mesh well with the original Duo-therm thermostats, it’s common for owners to end up replacing both roof units and the thermostats… obviously not a cheap fix.
There are several troubleshooting videos on You Tube, and you might check some out. Try the one by “My RV Works, Inc.” in Port Angela’s, WA… “Diagnosing An RV Air Conditioner That’s Not Cooling”; a bit talkative but thorough. Also is one by “RV Tech Tips”, “Dometic AC that won’t start. How to replace a start capacitor”, among many others that may or may not be relevant to your model, and that aren’t all particularly well done. Also check out:
https://www.doityourselfrv.com/rv-ac-repair/ .
Our Duo-therms may be before Dometic bought in, but the principles are the same. Just be careful on the roof and be sure to discharge capacitors before messing with them. If you don’t opt to do the test yourself, watching the videos will at least give you an advantage when discussing the issue with techs.
Joel