Engine won't start and no power 12 volt

2008 - 2551  Can't figure this out.  Hoping somebody has an idea of what can be wrong.  Yesterday ... went to go into the side door of the coach and reached down to switch the power on next to the steps.  Nothing- red light would turn on...inverter lights not on either... put the key in the ignition and only heard a couple clicks ... engine would not turn over.  Battery tester indicates 12.3 for Ford engine battery and both auxilliary batteries at 12.4.  Used Die Hard Jump Starter and the engine turner over instantly without hesitation.  Let the engine run for about 30 minutes.  Shut it off...waited 5 minutes and it started up again.  Today I just went through the same ordeal again.  No power ... what could it be??
All batteries have water,  posts and connectors are clean and tight.  Nothing left turned on inside the cabin and keys were taken out of the ignition.

When the engine doesn't start next time, use the jump-start switch that Phoenix USA installs by the driver's left knee.  You have to hold it in position while you crank the engine.  If not the switch springs back to the off position.  If the engine starts running, let go of the switch.

Then report back with your observations.