I agree with you. I doubt that the alternator, in good condition and properly wired, wouldn't keep the batteries in good condition.

We too have a 400+ service battery bank and a 65 amp starter battery, all kept in excellent condition by a Lucas 70 amp alternator. Yes, the alt. does run hot after heavy use of the services, but we've had no porblem over many years, a lot of which was full time cruising.

The alternator should be capable of out-putting it's full designed load and I suggest it's well worth whipping it off and having it tested. For example, Lucas Marine tested my 70 amp unit (it is a standard 'car' refurbished one, and I wanted to know it was ok!) and it topped out at 69.4 amps, which I thought was near enough!!

As others have said, it's sensible to check the wiring thoroughly, get the alternator checked and check the Sterling, before rushing out to buy a larger unit.