Replacing two 6 volt house batteries with one 12 volt

If you dont need a battery then a small lawn mower battery would work, 12 volts of course.
and of course you will need the battery at times at least for lights or when the power at CG goes out, or when pulled into a rest stop, but one 12v marine batt is also plenty.

strongly recommend getting the standard marine battery, cheapest is fine, though.

2 GC batts are ideal, but not if you will never need them