I am really skeptical that 12VDC power distribution in RVs actually saves power compared to a high-quality (hah!) higher voltage AC or DC system. 12V is absurdly low and you can’t easily lose quite a few percent in resistive losses even with fairly large cables, and those large cables are quite unpleasant to work with and rather dangerous.
The vast majority of what you're running from RV DC are things like lights, fans, phone chargers, and other cigarette-plug-adapter type devices. My RV has a 12V DC fridge. For anything more - particularly air conditioning - you need AC for sure.
My inverter-charger is connected to my batteries with 4/0 cable. That wasn't fun to run.
