Di-Directional Isolator

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?

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

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.

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

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

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 ?

yes a 'continuous duty 200amp contact relay/solenoid'

turned on/off by a 10amp toggle switch on the dash

best guess without more info
is you run the house batteries really low, take off down the highway and the alternator amps into the house batteries is too much for the contacts

these bi-directional are most often used in situations where the RV will be parked in place with shore power, and the bi-direct keeps the chassis battery charged by siphoning power from the house charging converter over to the chassis battery, so it doesn't go dead because of no alternator charging

MrWizard wrote:
yes a 'continuous duty 200amp contact relay/solenoid'

turned on/off by a 10amp toggle switch on the dash

best guess without more info
is you run the house batteries really low, take off down the highway and the alternator amps into the house batteries is too much for the contacts

these bi-directional are most often used in situations where the RV will be parked in place with shore power, and the bi-direct keeps the chassis battery charged by siphoning power from the house charging converter over to the chassis battery, so it doesn't go dead because of no alternator charging

in reality the Bi-Directional Isolator controls the Relay that connects and disconnects the two battery packs when either has a charging source and there is a 1.3 volt difference.
This one will not connect the house battery to the Chassis battery when the engine alternator is running at 14.3 Volts, and the coach battery is down to 11.0. But the system works normal in all other respects.

Isolator

MrWizard wrote:

best guess without more info
is you run the house batteries really low, take off down the highway and the alternator amps into the house batteries is too much for the contacts

We charge all night off the 50 amp External power, house batteries show 13.4 volts
as we drive the house battery voltage drops to 9.0 volts, because they are not being connected to the chassis battery pack.

There must be a better way.

Disconnect the ignition line and solenoid line and hook them together.
I suspect you have more problems though.