How to know if isolator is ruined...

So, I went out for the first time this summer. I had both batteries charged up and ready to go. I have an isolator and 2 batteries. The first day was fine. On the start of day 2, the stereo started cutting in and out. Turns out, the stereo battery was dead. So, I started the engine, swapped the 2 batteries, and we were good to go for the rest of the day. The next day, the same thing happened. I had to switch them BACK to the way they were the day before.

I looked at the isolator and it looks corroded. Is there a way to test the isolator before spending the money on a new one? I am pretty sure that's my problem, but didn't know if there is a way to test it first so that I know for sure.

ELECTRICAL TEST:
1. Engine not running: #1 terminal of isolator should read vehicle battery voltage. #2 terminal should read auxiliary battery voltage.
The "A" terminal may read from zero to 13 volts. The "E" terminal on 3A series isolators (group 2) should read zero volts, the "R"
terminal on "R" series isolators (group 3) should read from zero to 13 volts.
2. Engine running and alternator charging: #1, #2, and "E" terminal on 3A isolators (group 2) should read voltage regulator setting of
approximately 13.8 to 14.5 volts. The "R" terminal on "R" series isolators (group 3) should also read voltage regulator setting. The "A"
terminal on 3A series isolators (group 2) and “R” terminals on “R” series isolators (group 3) voltage should read 0.8 to 1.0 Volt higher
than the reading of the #1, #2 terminals and "E" terminals.
3. For 12 volt systems the "A" post should read approximately 14.8 to 15.5 volts. The #1 and #2 terminal should read 13.8 to 14.5 volts.
If the "A" terminal reads 13.8 to 14.2 volts the regulator may be sensing the alternator output rather than the main battery. This situation
needs to be corrected for proper charging of batteries.
4. Colored terminal indicates "E" post on group 2 isolators and "R" terminal on most group 3 isolators.


INSTRUCTIONS FOR TESTING A SURE POWER ISOLATOR WITH OHMMETER*:
1. Remove all wires from the isolator.
2. Using a needle movement ohmmeter RX-1 scale or a digital ohmmeter diode scale, hold the Red* probe on the terminal "A" and with
the Black* probe touch terminal #1 and #2, and the "E" terminal for 3A isolators (group 2), and the "R" terminal for (group 3) isolators.
A good isolator will show a current flow from "A" to #1, #2 and "R", and no current flow to "E".
3. Next, hold the Black* probe on the "A" and with the Red* probe touch terminal #1 and #2 (terminal "E" and "R", if used). A good
isolator will allow no current flow from "A" to #1, #2 or "R" and will show current flow from "E" to "A".
4. Hold one probe on the aluminum heat sink, being sure there is contact by scratching through the protective coating. Then touch
with the other probe, terminals "A", #1, #2 (the "E" terminal for 3A isolators [group 2] , the "R" terminal for group 3 isolators). A
good isolator will show no current flow.
5. Colored terminal indicates "E" post on group 2 isolators and "R" terminal on most group 3 isolators.
*On some import ohmmeters, the red and black probes are reversed for these tests.
**If using a digital ohmmeter, a diode scale MUST be used.

perfect....thank you ken. I will test that out.