Even if you failed electric shop, best take a volt meter with you and measure those batteries. I think they'll be 6V. They're probably in pretty bad shape so just in case take a gallon of distilled water and a battery filler. Plus you'll probably need to measure voltages on loose cables occasionally.
And of course you could just count the cells. 6 cells is 12V and 3 cells is a 6V.
Maybe take your camera and take some pictures and post them here?
BTW, house batteries should not be connected to the starter...that's the chassis battery's job.
The original owner may have wired it up that way because in most older RVs the converter only charged the house batteries and the chassis battery would usually die if you were parked on shore power for several days. Which is why most of us install a float charger plugged into 120V and connected to the chassis battery. Or some install a Trik-L-Start.