I found some information in the owner's manual that was contradictory to my intuition - and there has been some aftermarket electric work on my coach - so I plan to run some tests this weekend by unhooking one battery at a time to see what is the non-boost-button default power source for each of the following. But maybe you already know which is the default, or if it varies by year/model?
1) Dash Stereo/Radio/Speakers
2) Dash Map Light
3) Dash DC Outlets
4) Javelina Computer/Monitor
5) Leveling System
6) Dock Lights
7) Built-in bay tube lights
8) Heated mirrors
9) Mirror angle adjustment knobs
Look at your 12V schematic either in the Foretravel manual or download it. It will quickly tell you which battery supplies voltage to the different accessories. Best to use the one for your model/year.
Pierce
Most of those shut off when I turn off the main 12v house system in my RV.
Dock lights, mirrors, are probably engine battery.