Using fridge on 12V whilst driving -switch setting

Simple question (I think!)

I have a three way Dometic Fridge in my new MH. When travelling I will set the switch on the fridge itself to 12V to keep it cool. However, should I leave the MH master switch by the MH door in the on position or should it be swiched off whilst driving?  The MH handbook is silent on this matter (as in so many other things!!!)