Using the frame of an RV trailer for the ground
The main thing you'll need to worry about is the very large currents flowing.  A 3000W inverter on a 12V battery will draw up to 250A.  That means you need very low resistance wiring, and really good connections on all wire joins.
Bolting a lug onto a steel chassis may not be good enough in the long run, especially if the steel starts to rust around the join.
You'll want to keep the distance between the batteries and the inverter as short as possible, since the very large currents noted above will require very thick wires.