I think that the control unit disconnects if the battery voltage drops below a pre set voltage [the drop dead voltage]. I thought it was 10.8v but with a cheapish multimeter a reading of 11.3v may be near enough.
In any event the question has to be where is the 2v or so from the battery being 'lost'? At the 50A fuse or the terminal connectors?
What I would do is run a wire directly from the battery positive to terminal B2+ and see what happens. It may be prudent to insert a 20A fuse in the wire.
Just a question, are you absolutely sure the battery voltage you are measuring as 12.7v is the leisure battery and not the cab one?
Can you confirm that when on EHU that the voltage from the charger measured at B2+ and B2 - is the same as at the battery terminals i.e. the charger is charging the battery.
Thx again. Oh, wow, this time when doing the checks you suggested I realized what I hadn't thought about before, that I was checking the leisure battery at B2 on distribution box while the motorhome was plugged into EHU. When I check B2 without being plugged in, it is really low, like 3.65v.
You asked me to confirm I was definitely checking leisure rather than engine battery, and I can confirm that (it's a Numax DC25MF back underneath the bed whereas the other is an Exide Excell EB950 under the hood).
You made another suggestion: I haven't yet run a wire directly from leisure battery to distribution box. I could figure that out and get 'er done if that's the key to the secret, but I haven't gotten that far yet.
So it sounds to me like I've got two charged batteries (leisure and automobile) but am getting next to nothing from leisure battery at the distribution box...