Bob,
For something as fundamental as your main positive and ground cables you need to incorporate a margin. That's the wrong place to be getting a voltage drop due to an undersized cable under any usage conditions. You correctly identified the 100AH Battleborn current ratings and also the theoretical max draw of your inverter. Other heavy current draws include generator cranking and your automatic levelers if you have them; the slide draws less. All current to and from your house batteries will flow through that shunt and the connecting cable, so that would include any other 12-volt house draw you might have going on whenever you activate one of the heavy users.
On my 2019 2552, Phoenix wired the positive battery cabling with 1 AWG, and my negative cable to my Bogart Engineering Trimetric Monitor shunt with 1/0 cable. Of course, my Trojan AGMs potentially deliver higher current than the Battleborns. It's worth noting also that on the Battleborn site, their cabling recommendations are for 2 AWG up to 120 amps and 1/0 for anything over that (even short runs).
If I were doing your install on my unit, I wouldn't consider installing anything under 1 AWG, and honestly I'd just put in 1/0 and not have to worry about it.
Mike
