Battery Power Problem

[nancita]

97 Gulfstream Ultra

I have 2 new 6 v batteries ( in line) fully charged according to voltmeter, solar monitor panel, and test panel on my stove hood.
I am plugged into AC. The converter powers my 12 volt lights but not the refrigerator light or the Cig lighter socket on the side above the dinette.
The battery power seems to be intermittent during the daylight , fading and powering as it wishes.
There is NO battery power at night tho the batteries are charged......
I disconnected from shore power and the solar. Batteries are charged according to my voltmetre and monitor panel above my stove.

Could this be an isolator or relay problem and how do I diagnose?
Where is the isolator.....the manual is very vague?
I am mystified.....

[jpf]

there's an 'ATS' (automatic transfer switch) which is under the bathroom sink in my
5291. It receives AC from both the generator and AC shore supply and decides which should
provide AC to the rest of the coach. defaults to shore power (what the campsite provides).

I don't think that's the problem though, sounds more like a grounding fault.
power converter might be worth looking at. mine's below the kitchen sink.
it is basically the distribution box and all the fuses for both AC/DC.

just to confirm: 13.5-7v is the meter reading for a fully charged 12v battery.
12v is flat. less than that and you could have a dead cell also.