I've had alternators (plural) fail in the past, where a diode went bad and allowed current
to flow, draining the battery. So they definitely can and do fail that way.
I wouldn't use an "isolator" as those are simply more diodes and no matter how expensive they are, they also cause voltage drop and of course can fail in the same ways as alternator diodes.
A manual switch would be the most failsafe thing to install. A relay, a good robust relay, probably as good since relays are also pretty reliable. Assuming you can find one rated for enough power, in constant use, at an affordable price
. Otherwise? Another manual battery switch and
you just have to make sure it is engaged properly.
Since diodes tend to fail during heavy loads (making/breaking the circuit especially) it might be more effective to put an ammeter in the alternator output. Then just make a habit of checking it after you shut down. As long as it reads "00.00 amps" after you have shut down, the diodes have not shorted out, and it really should remain good until the next time. As a side benefit, you get an ammeter to read when the engine is running.