Thanks Touses and PeeBee,

So what I think I am hearing is that there isn't really a 'good' option for a single BMS for all three batteries, and the 'better' approach is to have a standard BMS for my Starter/In-car Aux, and then a second simpler isolator for my Aux in the trailer?

* Starter => BMS => Aux1 => Andersen => Trailer => BMS/simpler => Aux2

Expect this way, I'm still charging off my Aux only once it's topped up fully, and can still use the Andersen plug for other things from my Aux1...

I'll definitely check in with an auto elec., as this will not be in my Patrol, but a newer Ford Ranger, so I expect there will be some level of "Smart" charging involved (I just prefer advice from people that actually work on "real" cars hehe)