Remember the 12-VDC used inside your motorhome comes from your COACH batteries.
These batteries power the interior lights, the temperature control panel among a number of other items.
The batteries must be kept in good condition and fully charged. Check that the batteries have water in them and that they are not old or damaged. Then, make sure your Converter, which operates on your campsite 110-VAC connection, is running and charging the COACH batteries.
If you use a multimeter and measure across the battery terminals you should read; 1- around 14.5-VDC if the batteries are not charged and the Converter is running, 13.5-VDC if the batteries are fully charged, or around 12.5-VDC f the batteries are not charged and the Converter is not operating.