Newbie question re: fridge

Hi there, taking my first trip this weekend!

I have my 27' Class C 2015 Forrester plugged into the house power.

Couple of questions about the fridge:

1) When I turn it on, does it automatically start cooling using the house power or do I *have* to press the auto button so it knows what source to cool it from?

2) When travelling, does it automatically kick in to propane, is this advised or will it drain my propane tank?

Thanks for any help

Keith