In general this would be the difference between one battery and two batteries in parallel.
You move the ground connection to the last battery to improve the balance of the system. I show these going to a bus bar, but you could also consider the battery terminal a bus bar. That is not the approach I would take, but it is commonly done. A power post is also a very acceptable solution, the general rule of thumb is 4 connection to one post. The frame connection can be considered just that, a connection to the frame. It does not have to be the negative bus bar, instead it is just a path back to the battery.
The wire size for the frame connection needs to cover all the devices that are connected to the body for their path to ground. The wire from the bus bar to the battery needs to cover the current for all devices connect to the bus bar. I would generally use the same size wire for that for the battery to battery connection, although in theory the current load is shared across batteries. I also generally fuse the battery bank to bus bar connection, but that starts to go deeper than your original question.