Hi
Hi
This all looks perfect.
Yes, you can earth batteries to parts of the chassis/cab/seats. Just ensure the seats are earthed well.
Check how the seats are connected to the chassis, and ensure it’s a clean contact. If painted parts connect together, this might not be good enough.
Perhaps test with a multi meter.
You picture suggests it might be a metal strap that earths the seat? If so, that should be fine.
Great plan though, especially the dual fuses.