To answer the subject I will say this
I sometimes get flamed for saying "There are no six volt batteries in RV's" but, fact is, that is a true statement..
Oh, there may be systems, such as the one I have, where when you disconnect the batteries and remove them they are six volt (2 each) but the fact is once they are installed like this {Bat}t{ery} they become one big 12 volt battery.
So, the only thing to be aware of is this.. There are Batteries, batteries, batteries (this goes on a while)
Deep Cycle and Starting have slightly different charge pramaters, Marine/Deep cycle comes closer to starting, and should be considered a starting battery for most uses (It is **NOT** a Deep cycle by any stretch)
Maintenance free and flooded wet (And AGM) are all different as well.
So it is important that you not mix battery types
But voltages .. No problem there at all.