Your truck is in deep sleep mode when the interior lights won't come on. It happens to me every day. If you have the ford app on your phone, it should alert you when sleep mode happens.
There are 2 batteries. The 12 volt battery under the rear seat and the hybrid battery under the truck. If you touch the apps icon on your center screen and then hit power flow, you will see the location of the hybrid battery in the truck animation.
I can't speak to the longitivity of the hybrid because mine only has 10,000 miles. Mine is a 23 Lariat hybrid and I love it. The hybrid system is great.