Ok, a lot of the ideas and thoughts never occurred to me. Like I didn't know I could just add another run of 6 gauge wire. I think that'd be fairly easy to do. Can I run the battery neg to the frame with larger (or doubled) wire and use that as the ground to the charger? Would I gain anything by doing that?
I forgot to add our typical camping modes. It's a mix of dry camping (can't really call it boondocking since we're in a campground with amenities - just no power to hook into) and campgrounds with power. We can be at either for a short 2 day weekend but up to 2+ weeks at a time (this is where the gen comes into play). Lots of shade so solar isn't really viable, but I'm going to really pay attention this year at the available sun spots for a portable panel setup. We are very frugal with power usage. All LED lights, have a portable propane heater to take the chill off before bed and after waking up (not while sleeping). And, when we get home, the RV gets plugged into a 30A outlet and charged until we go out again.
Lastly, I went with the 75A charger because, that's what Randy (at BestConvertor) recommended based upon what I told him which includes that I may add 2 more 6v gc batteries (for a total of 4).
Thanks all!