House Batteries, external and internal, Question

[macstever77]

I have an Airstream Interstate "B" motorhome (built on a Sprinter). We plan to do quite a bit of dry camping and want to up our house battery from 100 amp hours to 300 amp hours by replacing the original house battery that is suspended under the left rear of the chassis in a metal box. There is not room for two batteries at this location.

I plan to put 2 AGM Lifeline 6 volt batteries, 300 amp hours each, in series: a. One in the lengthened original battery box; and, b. One directly above the old battery box, but inside the interior of the van just in front of the left rear door. AGM batteries tend to recombine hydrogen with oxygen inside the battery itself to form water and thus pose so little fire hazard that they are permitted inside airplanes.

My question relates to having one battery inside and one battery outside the vehicle. If there is a temperature differential between the two batteries, will it have a harmful effect when the batteries are being charged or when they are being discharged? I plan to replace the current stock charger-converter with a three stage charger-converter?

Thanks,

Steve