Should this be a 12V system or a 24V system?
Most of your items are DC and the inverter is 3000W which is on the cusp of whats viable/sensible for 12V. I have similar use but a 2000W inverter and I'm going 12V
Finally starting to install this system Purchased Product Specifications Battery – SunnyTech (badged Lithium Valley) 12V 200Ah Lifepo4 https://web.archive.org/web/20210130064931/https://sunnytech.co.nz/store/default/product-details/121/69/0/0 Solar Panels – SunnyTech 120W x 4...
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I do not know what size breakers/fuses to use, and if I have enough or if I have too many - thoughts?
The solar needs both cables to "break" or disconnect, other just the one
The battery fuse is important and should be rated for max system
Each time you change the spec of a wire you should fuse for that wire.
Where would grounding wires go in the diagram?
"Grounding" in vehicles can be a confusing term, its different from "earth" in a domestic set up. In a vehicle its typically used as the return path -ve back to the battery completing the circuit which uses less wire and can make things simpler. Getting a good connection through a modern van can be problematic though.
Any other feedback/comments/concerns with this setup?
Check the specs of the SCC youve shown, its solar input is limited plus your batteries aren't in balance with your panels ability to charge them.
You need to be aiming to recharge your batteries each "average' day (can be difficult in the winter)