If you have a double battery setup, it is probably 2- 6-volt batteries. This is often done because you get more current storage for your 12-VDC system. They are tied in series, and then there is a ground from the negative end of one battery to ground. The other end (positive) is connected to the disconnect switch.
That button next to your door is far too small to handle any current load at all, so it controls a larger solenoid which can handle the higher current when the button is used.
This would be your COACH battery system which provides the 12-VDC for your internal equipment including your internal lights.
Your COACH battery is under your hood, but it does also have a disconnect switch system so that your engine battery does not drain while the RV is in storage.
And then there is the AUX switch typically somewhere on your dash. Pressing and holding this switch connects the two 12-VDC systems together in case the engine battery is dead.