Ok, I think we have the answer, thanks for your help. But also some more questions!
Yesterday I reinstalled the inverter/charger and did some checking. The standby load consistently read 15 mA for the inverter, which means no problem. I have to confess with my 60 year old eyes in a dark cabin
I may have had my multimeter set on milliamps thinking it was on Amps and thus it looked
like a 1.5amp load. When I got the inverter home in better light it didn't show that sort of load because I could see better to stick the multimeter switch in the 10 Amp load position where it should have been.
Long story short, I think I just stayed away from the boat too long and the standby load bled the batteries down. Apart from all the angst it has been worthwhile getting to understand the systems better.
Now I know for instance that the inverter switch stops that residual load. When I leave the boat for a longer time I should switch off the inverter, something I never bothered to do before as I have to remove squab and reach up to find it on the inverter itself.
But this issue has raised more questions.
I have a BEP 600-DCM battery monitor and shunt and because am in the process of installing a solar panel, have taken on board the necessity of wiring
the negative side of solar controller to the load side of the shunt so that the monitor can record
it's input. What I have found, which I can't figure out why it should be wired this way since everything else seems to have been done really well, is that the
negative of the inverter/charger seems to have been wired directly to the house bank negative instead of the load side of the shunt. Is there any reason why they may have done this? It means that the input from the charger side is not accounted for by the Battery monitor when shore powered. So how can it accurately calculate state of charge when one of the charging systems is not monitored? The monitor shows charging from the engine alternator through a Next Step smart charge regulator, and also will show the solar when I hook it up, just can't understand why they wouldn't have wired in the Inverter/charger on the load side of the shunt also.
I am reluctant to change things because when I bought her, the agent suggested I don't let any old hacker (me) monkey with the sophisticated electronic systems on the boat.
The other thing that remains a mystery to me is how the two alternators are wired. Do they both get regulated through the Next Step regulator? Does only the house bank get charged through the smart charger regulator and the other alternator use internal regulation for charging the starter battery?
Also, since changing batteries from 220 Ah up to 232Ah (meaning I have theoretically 464 Ah capacity) I find that when I switch on main isolating switches, (house and starter) the monitor shows an impossible (to my mind) one or two amp charge on the house bank), with nothing running to give charge. Is it an anomoly with not recalibrating the new Amphour capacity? Ie. is the monitor fooled somehow, or is there one bank draining into the other?