The battery icon shows a lightning bolt when it's charging. Of course, the battery icon does not display if the tablet is off, or the battery is so low it can't start.
If your camper has had power for over 24 hours and the your wall tablet remains off, it could be turned off: The Controller tablet can be turned back on by pressing any of the Light buttons on the Control Panel below the tablet.
If nothing happens, you may have success by pressing the reset button with a paper clip.
If this fails, you next option is to remove the tablet from the wall and use a phone charger overnight. There is an on/off button along the side and you can confirm it boots before reattaching it to the wall.
If all the above still doesn't turn on the tablet, you have three options: Replace the battery (
see this forum post), replace the tablet (contact your dealer, or try to find another 7" android tablet that will fit in the housing), forget the tablet and just use your phone.