The chassis ground goes to the neg bus and then to the load side of the shunt. #2 is likely plenty for that since the usual RV loads use the frame for part of their neg paths, and the higher amp items you have don't.
You need the fatter wire to go from the shunt to the battery bank since that side of the shunt carries the total of all the amps coming to the load side of the shunt which can be from a number of different size wires.
Also the wire from the neg buss to the load side of the shunt would be fatter since it could also be carrying the total amps. Might still be some loads going to the load side of the shunt but not via the bus, don't know.
The subject of this thread is solar wiring, which was not mentioned.