Thanks so much for comments and suggestions.
Lefty, your links are most interesting and useful. Much appreciated and I'll get into the Arduino with these as a basis. I do realised this is an Arduino board and NOT a campervan one, hence the nature of my post.
To answer other comments/suggestions.
I bought the (German) vehicle new in Jan 2012, factory configured to my specs, and had it professionally converted in Australia before I saw it. The intelligent charger for the house batteries can only operate when the vehicle alternator is actually charging and in any case it will only pass current to house batteries (two, high capacity) when voltage is above about 11 volts. (ALL circuits in the camper side of the vehicle had been switched off for a week and the house batteries were at 100% charged state at the time I found the problem.) The local service agent checked the vehicle electronics/electrics and could find no fault. The vehicle has the Canbus system. Battery was tested too and found to be fine (fortunately). They were persuaded this was a reasonably warranty issue and didn’t charge the $175 fee but they weren’t prepared to look further. Given that the issue has occurred only once (or possibly twice) in the 10 months I’ve been driving the vehicle I am probably not surprised they didn’t find anything.
I said in my post the voltage drop was overnight. It might have been a couple of nights/day. I am capable of measuring voltages and current draw etc, but given that this has occurred so infrequently and that the car often sits a week or two unused with no issue, individual measurements seem unlikely to help much.
I suspect I would void my 3 year warranty if I put an isolator between the main battery and the vehicle. I’ll ask.
I have driven my other camper for 28 years and never once had such an issue!
Folks, thanks for your interest.