'01 Pleasureway 19' wide body, coach battery not charging from motor

[Gup]

Hello forum,
I have about 22,000 kilometres of experience with this rig in one year of ownership. It is an '02 wide body with 110,000 kilometres on it, a Dodge one ton chassis with Excel TD floor plan.
The coach battery was replaced last September with a deep cycle RV battery,
before embarking on a 12,000 kilometre trip to California.
Afterward, I removed both the batteries for winter storage and maintenance.
The coach battery functioned normally for the first 1000 kilometres this season then the 12v fridge drained it while on the road after about 5 hours. I learned I have an alarm that warns me of this! That took a few minutes to figure out.
So, it charges normally plugged into AC but not from the alternator.
Full disclosure: I switched the fridge to propane and preserved the aux battery for the roof fan and 12volt water pump.
Approx. 6 hours further down the road while stopped for fuel, the ignition switch was dead. No dash lights, no solenoid click, nothing at all - except the chime to warn me of the key in the ignition and the driver's door open! I was baffled. I checked under the hood for loose connections at the battery and isolator terminals - all tight.
Next, we heard two very loud 'electrical' type thumps in the rig about 30 seconds apart, still while parked at the gas pump, which were impossible to locate. A sound similar to what you would experience through your home speakers if you were to switch modes without turning down the volume (tuner to phono, for instance).
A few minutes later I again turned the key and the motor started instantly as per normal, all dash lights functioning normally, and has ever since. That was at the end of June on a 3000 kilometre return trip to the East coast. None of the symptoms has reoccurred, everything functions perfectly except the charging of the coach battery while underway.
I have checked the breaker, it appears normal, no exposed tabs to reset.
I attempted to check the isolator as per instructions found online by the manufacturer and it passed the tests (which doesn't completely rule it out as the culprit).
Could there be another link in the chain? A converter problem? The converter seems to work as designed. It charges the coach battery, runs the microwave and roof air. The 12 volt lighting is LED now, so quite efficient, but I'm assuming it is still working off AC power when plugged in.
I would love to hear of similar problems and solutions or suggestions from the experienced amongst you.
Thanks,
Gup.

[hbn7hj]

Just a guess but there should be two breakers on the power line from the isolator to the battery.
One around the fire wall and the other at the other end of that line closer to the battery. Try to find the second one and check if it is open.