RV Repair: 6v batteries in series w/power jack, flashlight batteries, 6v batteries

Question
I'm getting ready to hook up again for another season....the dealer sold me two used 6v batteries that they hooked up in series and also to the power jack which was all disconnected for storage.

This is what I think needs to be done but just want to be sure before I proceed

-take trailer lead and hook up to positive on Battery A then connect from Negative on Battery A to positive on Battery B then connect the other lead of trailer wire to negative on Battery B, pretty sure that's right but.....just not sure where to put wire for power jack?

Also how do I check if these batteries still have juice, I've been charging them on a battery charger overnight approximately every 6 weeks....once fully charged how much time with lights would I have and how do I gauge this....last season I was dry camping and the batteries which I thought were fully charged gave out thru the night and the CO2 alarm went off at 4am.....I had to connnect to the truck until I was permitted to use generator from 10am to 4pm....I also wasn't 100% on what setting to put the charger on when charging....if I'm charging two 6v in series that would be 12V correct? So is that the setting on the charger to be selected?

Answer
Gerard, First off why are you using a battery charger for the batteries? The coach should have a converter in it to charge them. As for the way to wire you have it correct. The jack wire should be hooked to the outgoing positive side. The one that has the trailer positive wire on it. The reason that you connect one negative to the other positive is because a 6 volt battery has only 3 cells in it and a 12 volt has 6. Each cell puts out a little over 2 volts. So picture even flashlight batteries sitting side by side and that is basically all you are doing by connecting them this way. I hope that is clear enough to understand. You just made a 12 volt battery inside 2 cases not 1. As for how long they should last, well check the amp rating of the battery and then the rating of what you use and you can figure it out pretty close. To try and understand even better go to any battery supply store and they should have a chart that they can give you so you can do just that. You should get longer usage from 2 six volt batteries than 2 twelve volt batteries. The cells are usually larger and the plates thicker. I hope this helps, Bill