You wrote in the first post "Last weekend I replaced the isolator and it fixed until I tried to start the engine
.". What do you mean that it fixed? What tests did you do to determine everything was working right after replacing isolator?
As a minimum, please do the following tests:
Disconnect any shore power
chargers, wait 30 minutes. Then measure battery voltage before starting engine, with the battery switch in OFF position. Once you get it started, then test voltage at alternator
output terminal, and on each side of the isolator, and at each battery terminal with the switch in the Batt1, Batt2 and Both positions. For these tests, it is essential that you connect the black lead of the multimeter to the same place each time, preferably the alternator case or it's ground terminal.
Also please let us know what is the battery size, isolator type and model, and the alternator type (at least, is it a standard automotive size or something larger) and the voltage regulator
type (external or internal to the alternator). Post results here, you will get better help.
BTW if you heard a pop, I would bet there is a bad connection or a short circuit somewhere.