12v power with engine running

This is potentially a stupid question but easy to answer. We havent had our motorhome long.

i have a neuvo and when hooked up or an battery the electrical systems all work fine.  As we haven't used it for a couple of weeks I had the engine running for a while. when the engine is running the the  12v electrical systems like the lights don't work.  Is the normal or not? The fridge is fine on 12v when engine is on.

thanks.

As it should be. A relay cuts off the 12volts to habitation when the engine is running. A relay also brings 12 volt power to the fridge when the engine is started.
Al