6volt batteries in older Northern Lite

I suggest just doing the two larger 12 volt batteries in parallel. Factory puts in too small of 12 volt batteries and larger 12 volts do fit. It much simpler saving the big wires for a future generator.

Some well planned solar panels will be a big plus.

Getting rid of the old convertor would be number one on the list. You need a good 3 stage charger to not ruin any new batteries. Which those old style convertors do damage.

"from a load balancing standpoint"..
Which is my assumption.
To achieve the same with 12v batts in parallel requires a 3rd cable, otherwise 1st battery sees more of the load & charge. Greater the distance more it affects.

With 6vs and 2 existing cables between-done. Distance really not an issue. Hence easier in opinion.

Well actually it’s exactly the same effort to wire them. It’s still two cables you just connect to different posts on the battery.
With 2-gauge cables you should be fine.
Two 6s in series is more advantageous in this situation from a load balancing standpoint, since all the load has to pass through both batteries in a single electrical loop.

Agree, 2 6v easier to wire with 2 cables

Just ensure cables marked/labeled for fool(me) proof connecting.

In series you should have no issues.

I don't have a truck camper, but I will make a comment about distance between batteries. I have two separate battery compartments that are about ten feet apart. There is a slight voltage difference during discharge and charge between the two, about .2 volts, with the distant battery having the lower voltage. I don't have 2 awg though, more like 6 awg. With 2 awg I don't think that any difference will be enough to worry about.

I wouldn’t worry too much about the distance.

Why are you apparently only concerned about two 6 volt batteries and distance?