Advice about fridge power for a newbee pse.

We take delivery of our first motorhome in Oct but have limited experience of using a hire motorhome.  When we have hired, powering the fridge is obviously no problem on site with EHU or on an Aire where gas is required, or when driving around when you switch to 12V.  My query is for those times when you take a short trip out to visit somewhere for say 4/5 hours.  Should you automatically switch over to gas or can you just remain on the 12v as if you were driving around?  If its the 12v is it the leisure or vehicle battery that is being used?  Sorry if this is a bit basic for many of you out there!!