No 12 Volt appliances working with engine off and no 110
We have a 2004 Ford Sportsmobile that we bought used and have taken a few road trips in it. This is no doubt a raw beginner's question, but it seems to me that the 12 volt appliances should work with the van parked , engine off and no 110 plugged in. The house batteries are showing between 13-14 volts on the display, but the 12 volt appliances- interior lights as an example- do not come on unless I start the engine or plug in the 110.
What am I missing ?