AKat777, I have lost track of your design intent. Do you have no 12V house batteries at all? If so, you are coming to an unusual design solution, but if it works for your mission that's all that matters. Most people in your situation would take 12V from the starter battery for occasional and minimal loads. Even one ceiling fan running all night while you are asleep is not a minimal load, so you might be better off not doing that. Not being able to start the truck in the morning would be a bummer. On the other hand, plugging the starter battery into a proper 3 stage charger with say 10A capacity would keep it fully topped off at all times and replace whatever your fans draw. That way you would still have 12V when not plugged into shore power for fans and other things as long as you don't draw too much for too long. Such a charger migh also have anti-sulfation technology and extend the life of your starter battery, especially if it sits for long intervals without running the engine. Having full time 12V might be handy for emergency lights, an entry light on the outside for finding the lock with they keys when it is dark, CO detector, or little accessory things like a cell phone charger, etc.
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