How many plastic caps do you see on the top of these batteries you see?
I agree, that the one under the hood is for the automotive part of starting the engine (should have 6 caps = 12 volt battery) also know as your "chassis" battery.
The ones under the steps are usually your "house" batteries to run your house lights, roof fans and such, requiring 12 volts to operate.
Each 12 volt battery will have 6 plastic caps (my rig only has two 12V "house" batteries located under the steps) but like was mentioned, your rig may have four 6 volt batteries to do the same (3 caps = 6 volt battery)
Ken and Terry
2010 Georgetown 330TS being pushed by our
2017 GMC Terrain toad (4 wheels down)