Inverter negative power wire should goto battery attach at shunt where frame ground is
Everything negative in and out of the battery should be attached at the out side of the shunt so all current goes thru shunt
Solar negative, converter negative, inverter negative
All power has to go thru the shunt
Electrically it is possible to use the RV frame if done correctly
But it puts more connections in the negative circuit and use's the steel RV frame for inverter power instead of copper wire
So it better to use wire for the inverter negative all the way to the battery
Just like for the positive
Just get the connection on the correct end of shunt