1. There could be a short in the AC wiring on the PV side of the inverter.  I would first check the RSD manufacturing date.  We want to see an RSD with a manufacturing date after March 2017.  Typically, some of the oldest RSD's would throw a ground fault alarm, not an ac trip.  So this is not likely the problem.
  2. You have one ac circuit running from the inverter to the RSD and another ac circuit running from the inverter to the POC breaker (these two circuits are bused together at the inverter).  Disconnect the RSD ac conductors (L1 & L2) and use your meter to see if the RSD ac circuit is shorted.  If so, pull new wires.  Check the circuit again.  If the short is still there, replace the RSD.  If not, good.  Re-assemble the RSD ac circuit. 
  3. Now, "walk down the ac circuit".
  4. Disconnect the inverter from the ac disco. turn on the breaker.  If the breaker trips, you got a short in the ac output wiring or in the PV system ac output switch.  Check for loose connections and abrasions on the wires (in the wire bundles and at the conduit fittings) inside the ac switch.  If nothing is loose or abraded, disconnect conductors from the breaker and pull new ac wires through the conduit being careful not to nick the insulation.  Put back together again.  If the breaker doesn't trip, good.  turn off breaker. 
  5. You can check the wires between the inverter and ac disco by eye-balling them since these two components are typically right next to each other and the wire run is only a couple of feet.
  6. At this point, I like to change out the breaker.  It is possible that the breaker is fine now, but when the inverter (ac output conductors) gets some load, it pops.  So I like to put in a new one, just in case.
  7. I also look at the inverter alert log (I will see NO GRID alarms because the breaker keeps tripping).  I would like to see if the breaker trips right in the morning (low PV output) or in the afternoon (high PV output).  (NOTE: The inverter clock must have been set correctly).  But I might also see an ac over-voltage alarm.  This will happen when the voltage rises at the inverter ac output terminals.  And it's typically caused by too much resistance in the ac output circuit conductors.  The ac conductors were sized a little too small. 
  8. I look at the wire run.  If its a relatively long distance and the installer used #14 or #12 wire to get to the breaker, he might try pulling larger wire.  The smaller wire and high resistance in the ac output circuit may be causing the breaker to heat up.

If these strategies do not work, and the inverter keeps tripping the breaker (especially if the breaker is tripping almost every morning), I would then swap out the inverter.