re :
> The Dometic frig in my 85 Vanagon runs great with gas or AC current but will
> not cool much with battery current. Why?
>
> Pretty normal in my experience.  - Works great on Propane and 110 AC , but is semi-useless when on 12 volts and driving. Might be an issue of fridge age and how clean or not not clean  things are in the frige systems.
> One tactic is to cool it down really well on AC befoer heading out driving with  frige on th 12 volts. That might help.
> And as stated it's wired so if you are driving with fridge on 12 volts, as soon as you shut off the engine 12 volt power to the fridge drops out, via the relay under the seat.
>
> Rather clever that relay ...engine has to be running, and alternator charging, before it makes  12 power  available to the fridge.