Just think like this:
When properly installed in an RV (Motor home or trailer) There are NO SIX VOLT BATTERIES,, Only 12 volts that come in 2 pieces for ease of handling.
So you want to hook up like this
{Bat}t{ery}
-(Battery}+
And yes, that will work I saw posts about efficiency and other yammer warnings, For the most part, those folks do not know what they are talking about. They will work fine.
There is, however, one slight danger.
should the 12 volt short out (Shorted cell) there is enough power in the six volt pair to make it go "pop" and spray acid all over the place.