I'd start by replacing the breaker. It's cheaper and (IMO) more likely to be the problem.
If the AC won't run properly on a fresh breaker then it never did, not even for the PO. But before I bought a new AC I'd take a hard look at the wiring.
There are four bolts that hold a roof AC to the vent orifice. If you remover those you can press the AC up and on to the roof. Somewhere at the side of the vent orifice you'll find the AC hardwired in with thumbscrews. Before I went out and bought a new AC I'd try powering it directly via the hardwire connection, just to see.
Also, have a look around. Is the water heater on electric? The battery charger running? These things are FAR more likely to be the problem. Maybe your batts are dead and the converter can't stop trying to charge them.
