Using different batteries and solar chargers questions

12v batteries are parallel - positive to positive, negative to negative.
6 v batteries are series - you make a 12v battery by connecting trailer negative to battery 1 negative, battery 1 positive to battery 2 negative, battery 2 positive to trailer positive.

Better diagrams can be found here:
http://www.marxrv.com/12volt/12volt.htm

You can usually avoid the core charge by giving them an old battery. It doesn't have to be the same type, but they generally count power sport batteries as half a core.