New dual 6v's. Need some advice plz

Distilled water "eventually"??? Get it now. People have gotten spoiled the last 40 years or so with maintenance free batteries. 6 volt golf cart batteries need frequent re-watering. You might want to consider the watering systems they sell, especially if the batteries are in a location where looking in the top isn't easy. I'd install them now and let the RV's converter do the job it was intended to do. The upside to the 6 volts is that if you keep them watered and the terminals clean they will usually reward you with 7-8 years of service. Our previous coach had a WFCO and we had no problems with batteries being undercharged.
https://www.amazon.com/Trailer-Electrical-Battery-Watering-Stystem/dp/B00MJUP1SA/ref=sr_1_12?keywords=battery+watering+system&qid=1551472761&s=gateway&sr=8-12