Battery Bank

Wire each pair of 6 volts together positive to negative then wire the three sets together positive to positive and negative to negative. Wire ONE lead on the negative side to the RV negative and one positive lead to the RV positive lead.
Each set of 6 volt batteries is made into a large 12 volt then you wire the 12 volt sets together just as any other 12 volt batteries to maintain 12 volts as the output.

So like this but with extra two batteries obviously. I wanted to avoid the pos/neg busses but if this is the only way...

Mena, you keep showing pictures of that other guy's new Paramode. :)

As BFL mentioned, you need to put all the negatives on one common bus and all the positives on a different common bus.

This is how I did it. Those are Blue Sea bus bars.
EDIT: You can see the shunt for my Victron battery monitor connected to the negative bus bar. The "other side" of the shunt is connected to chassis ground so I get everything the comes and goes to batteries.

I have that problem. I have four 6s and two 12s. I made two pairs of 6s and paralleled the 12s as another 12, so now it is three 12s.
Six 6s would be three 12s too.

Now you want proper balancing for three 12s which is tricky. You need to link each 12 to common busses with equal length wires from each 12. Too difficult for me, so I treat it as two 12s and just put the loads on the outside ends. The Trimetric shunt screws it up, but you can use the shunt's battery end as the buss for the negs to get close to the right answer.