ANY WAY TO TEST THE ISOLATOR?

Fire up the engine. Read volts from terminals to ground. The terminal connected to the alternator should read the charging voltage for your alternator. Around 13 to 14 volts. The terminal to each battery should read around 1/2 volt under that. The voltage loss is characteristic of silicone diodes. That checks charging current flow.

There are two ways to check the isolation in the other direction, one easy, one more involved.
The easy way is to use your coach battery at night and read its voltage in the morning. It should be below 12.6 volts. If it is 12 volts, you used half of it. The chassis battery should stay at 12.5 or 12.6 volts. The other way is to shut off the engine and disconnect the wire from the alternator terminal on the isolator. Use a 12 volt test light from ground to that terminal. It should not illuminate. Then disconnect chassis battery from the isolator and connect light from that terminal to ground. It shouold not illuminate. Connect chassis cable to isolator and disconnect coach battery from isolator terminal and connect light from that terminal to ground. It should not illuminate.
Ther are at least two other ways, but this is a real world way, under load.