Help, don't understand how the 12v system works.

There is a way to make all the 12 volt stuff work endlessly when on shore power, but it requires a device called a converter. Your Econoline conversion might or might not have one. And if it does, it might not be working.

It is the function of the converter to recharge your house battery (not your starting battery, however) from shore power and to manage the 120 volt and 12 volt systems. It is usually where the fuses/circuit breakers for everything are located.