I am am finding the issue confusing.

I would opt eliminate the Battery Isolator Completely (as previously suggested). The Alternator is going to charge all of the Batteries or not charge all of the Batteries (your gauge should tell you if you are having a problem).
With the above way there is no guessing if the Battery Isolator is working or not.

Yes when you use the LIft it will draw current from all the Batteries.
However, there still should be plenty left to start the Bus since you would be starting on all of the Batteries.
I suppose that a Manual Disconnect Switch could be installed to prevent discharge of the Starting Batteries so you would only draw current from the Lift Batteries. But, that would add complication and "Human Error" if the Driver failed reconnect it.

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84 300D, 82 Volvo 244Gl Diesel

It appears that the battery isolator has already shown to be bad.

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Gone to the dark side

- Jeff

Correct.

I just ordered the 225 amp relay . . .

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82 240D Manual 277K and still rolling!
02 Volvo S60 AWD For Sale