Quick couple of questions for SOK 206a battery setup

[SniperX]

I am replacing 2 AGM batteries with 2 SOK 206A batteries. I was originally going to use one SOK battery, but I quickly realized I was limited to 100A by the BMS, so now I am using 2 of them. This is a 12v system. I have 3 of the 12V SOK 206A batteries available, but not able to put them all in one place (limited space) and not sure it is a good idea to have 2 batteries in one place and the third about 2 1/2 feet away from the other two.
I have 400W solar. Rewiring my whole system because the way it was wired by the person I bought the camper from is wrong.
If I wire those 2 12V 206A SOK batteries in parallel, should I use 2/0, 3/0, or 4/0? I don't mind going bigger for peace of mind. I do not have an inverter with the system currently.
If I want to add one later, would it be better to just go ahead and use 4/0?
Would you use a class-T fuse for a 12V system? I know that the fuse size depends on the wire size, so this is where I am stuck and need some advice.
I am adding a cut-off switch for the battery, a cut-off switch for the solar, a fuse for the battery, and upgrading all the busbars. All wiring is under 3 feet roundtrip to each other.
I have 4/0 wire available but could order the right size if that is not a good idea to use the 4/0.
I am pretty new to all of this, but I want to make sure I am doing the right thing for the 12V since I know it needs thicker wire than a 24V or 48V system.

 

[PlanetExcellent]

I also have 2 200aH lithium batteries (different brand) and I do have an inverter. I went with 4/0 cable from batteries -> Lynx bus bar -> inverter. I did the calculations for the length and the maximum current load (which I'll probably never actually draw) and I think it said 2/0 was sufficient. But I'm the cautious type so I chose 4/0 to give myself a safety margin.

 

[SniperX]

I also have 2 200aH lithium batteries (different brand) and I do have an inverter. I went with 4/0 cable from batteries -> Lynx bus bar -> inverter. I did the calculations for the length and the maximum current load (which I'll probably never actually draw) and I think it said 2/0 was sufficient. But I'm the cautious type so I chose 4/0 to give myself a safety margin.

Thanks for the response! I think I will just use the 4/0. I would rather know I oversized to be safe. This way, I can add an inverter later too. Did you use a class-t fuse after the battery or a fuse after the batteries?