The isolator is just getting itself turned on with E terminal it appears from the schematics as the Roadtrek one shows the relay from the ignition putting power to the E terminal from the house battery. Inside the box it is probably doing something simple like turning on a connection from E to A so the alternator can see that there is a battery in the system if the regulator is checking for that in the alternator.
It appears that the test procedure in the link, which is the one you mention, does seem to look you are testing that A to E connection with key on, engine off, vs key off only.
"1. Engine not running, terminal B2 should read system battery voltage. Terminal B2 should read
auxiliary battery voltage.
2. The A terminal may read 0 to 10 volts if using a digital ohmmeter. This may be due to the
capacitor in the alternator. Ground out terminal A momentarily and retest, voltage reading should
be 0.
3. The E terminal should have 0 volt reading.
4. Key on engine not running, terminal E should read system battery voltage, terminal A should read
system battery voltage or may be as much as .6 volts less battery but no more. Battery post of
alternator should read battery system voltage or as much as .6 volts less than battery but no more.
5. For 12 volt systems the A terminal should read approximately 15 volts. The B1 and B2 terminal
should read 13.8 to 14.2 volts.
6. If the A terminal reads 13.8 to 14.2 volts the regulator is sensing the alternator output rather than
the main battery. This situation needs to be corrected. for proper charging."
It appears that the test procedure in the link, which is the one you mention, does seem to look you are testing that A to E connection with key on, engine off, vs key off only.
"1. Engine not running, terminal B2 should read system battery voltage. Terminal B2 should read
auxiliary battery voltage.
2. The A terminal may read 0 to 10 volts if using a digital ohmmeter. This may be due to the
capacitor in the alternator. Ground out terminal A momentarily and retest, voltage reading should
be 0.
3. The E terminal should have 0 volt reading.
4. Key on engine not running, terminal E should read system battery voltage, terminal A should read
system battery voltage or may be as much as .6 volts less battery but no more. Battery post of
alternator should read battery system voltage or as much as .6 volts less than battery but no more.
5. For 12 volt systems the A terminal should read approximately 15 volts. The B1 and B2 terminal
should read 13.8 to 14.2 volts.
6. If the A terminal reads 13.8 to 14.2 volts the regulator is sensing the alternator output rather than
the main battery. This situation needs to be corrected. for proper charging."
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