It is typically much morefficint to charge those evinces directly from 12 vdc. Adding 12 vdc outlets is as easy as anything, just ensure that they are people fused.
Most inverters are very inefficient with small loads. It is only when they arre loaded near capacity do they achieve thier advertised eff numbers. The exception (that I know of) is the Morninstar Suresine 300, a great little inverter, not often used in Rv applications...mores the pity! I run my entire off grid house on one.
As for the number of batteries, and the total Ah capacity. It is usually a trade off between total Ah delivered oer the life span at what cost of say a 200Ah bank, versus more (but less deep) cycles with a larger bank.
In RV applications, most batteries don't die from using up thier designed number of cycles, but rather they are "killed" from over drawing, under charging, heat and vibration. Is use the same calculus when I design off grid applications. If twice as much battery lasts less than twice as long, in total, and in total Ah delivered, then it is cheaper to go with the smaller set, draw them further each cycle, and place time sooner. As I said, in an Rv application they ease likely to be killed early from other causes, so going with the small bank, monitoring performance with something like a Tri-Metric, and then deciding what to do with the next set. That would be my suggestion.
Icarus