You have a single chassis battery, on my 2016 it is right in front of the passenger under the hood.
You have 2 house batteries. On my 2016 they are in the first storage bay behind the entry door.
You should have an emergency start switch that will tie the house batteries and chassis battery together to start the coach if the house batteries are good and the chassis battery is low.
Ted & Melinda
2016 ACE 27.1
2016 Chevy Sonic Toad
2020 Chevy Colorado Z71 Trail Runner Alternate Toad