I recently bought a travel trailer and the batteries are dead so I need to replace them. Currently it has 2 12 volt deep cycle batteries. My dad has a 6 volt battery and I was going to buy a new 12 volt deep cycle battery. Is it ok to use these together to power the lighting and fridge in my camper or will it only put out 6 volts of power which would mean buying a new 12v battery is pointless? I don't know a whole lot at all but I need some help quick if anyone knows the answer to this. Thanks!