Thanks for the replies. It's going on a inline 4 turbo gasoline engine. It's just seemed easier for me to add a second alternator and seperate the wiring completely instead of reengineering 1980s wiring for way more load then the stock 65amp alternator was ever intended for. Basically, I either run a upgraded alternator, or a second in which I have no problem fabricating up a mount.
I still do need to do some research on a dcdc charger.
Step back and reconsider what you are doing. You are talking about adding extra battery and extra generation. Why on Earth would you want to deliberately isolate those systems and throw away potential options of combining resources or cross-linking things or leveraging other options. Especially when you are considering sinking premium money for components.
I'd want a combiner / isolator option and maybe even some rooftop solar too, for passive power, which can extend the period of time between when you need to run the vehicle.
too, most alternators don't generate anywhere near their rated capacities at idle / near idle. So you'll need an ALT that's purpose built for that, as for police/emergency vehicles.
Properly configured, your batteries ought to be fully charged when you get where you are going. Then the issue is more to extend their useful charge without havign to run your vehicle often. Or you change your movement plan so you drive a good bit at moderate speeds between 1-2 day campsites. That way you are topped off before each stay.
If the plan is boondocking / extended stay, I'd say go more solar, instead of sinking money into an expensive battery and second / improved alternator solution. Throwing a lot of money at something is not always the answer.
I've got dual matching SLA batteries and a solenoid isolator / combiner and rooftop solar. I chose the matching 'stock' batteries so if the Starter batt fails on a trip I can just swap them. I've got your desired 100AH capacity and 5AH in solar and my trips in the desert southwest involve a lot of miles covered. It works for trips of a few days' duration. My whole setup cost less than that 100Ah LifePO4 battery alone. I'm not claiming poverty is a virtue. Just saying take a step back and look at what you are trying to really accomplish overall then pick the pieces that support that, instead of chasing one costly addition after another. Buying the most expensive parts is not a magic solution. In fact you are finding out that the fancier batteries aren't fully charged by most factory vehicle charging systems and are now having to double down on costs to make it work. That's what's called a 'type 1' error, an error in conception of design.
DiploStrat is right, go for the direct (no DC pun intended) solution and let your factory charging / battery system do its thing. And stop shoveling money at induced problems.