Grit Dog, Converting RV house batteries to Lithium is not cheap or simple, you will need to change the converter charger to a lithium type. Lithium batteries need to be protected from low temperatures. It's doable.
Rick Jay, I'll try series connection for 24 volts without using converter. I will need a 24 volt charger or can charge each 12 volt battery separately with 12 volt charger.
Connecting two batteries in series and getting the same capacity/duration just doesn't seem right. BTW, my moniker on RC Groups forum is "E-Challenged" for good reason.