Di-Directional Isolator

All works normal, except, the engine alternator will not charge the house batteries when driving. the chassis battery charges normally. the external 50 amp connection charges the house battery normally.

Isn't there a manual alternative ?

bi-directional is a relay, whichever charge side gets to above 13.2v
Pulls in the relay to charge the other side

Please describe the problem you have with the isolator-why do you have to replace them.

The most likely cause is a loose connection. They take a big load, and are rated to take that load, but if you have a loose connection that load will look like a big resistor to the conduction of the current demanded. It will overheat, and then heating affects the wires and they will burn and cause corrosion and then not conduct like they should. This will lead to failure. Take the connections apart, clean them up, and put them back together tightly.

And that does mean all ends of the connections, not just at the isolator.

Mine has not failed in 14 years.
Like above you need to find the root cause.

Start trying to figure out why. Are you getting voltage spikes? Is your alternator bad? What is causing it to fail?
There are 39 dollar BiDirectional isolators and there are 79 dollar ones. What is the difference?