What to use for ground in a complete Roboteq system with BMS, FBL, PC, battery ?

Consider a robot made mostly out of metal, and a complete Roboteq configuration consisting of a BMS with a 48V battery pack and charger, FBL, and BLDC motors with 24V brakes
The motors have a frame ground. The FBL manual says to be careful not to introduce ground loops, especially as far as the FBL digital I/O connector is concerned. And the BMS makes the distinction of "earth" vs "ground". So in this complete system, we have:

Charger chassis/earth
Charger output minus
Battery pack out/BMS input minus
BMS output minue/FBL input minus
BLDC motor frame
PC/computer system 12 V minus

So among all these options, what should the metal parts of the robot be connected to ? This is what I'm assuming:
- charger earth connection tied to the motor frame grounds but isolated from all the DC grounds
- battery pack minus only connected to BMS input, not connected to any other ground
- BMS "load" output/FBL input is the main digital ground, and all auxiliary power supplies like the 24V DC for the motor brakes and the 12 V DC for the computer have *BOTH* their inputs and output minus connected to this main digital ground.

In this case, the issue I have it what happens to the ground on the RS232 connection from the FBL to the computer. Again, the FBL manual says to watch out for ground loops and *NOT* to introduce a path from the I/O connector's ground to main ground. So does this mean the RS232 connection would not be feasible unless an optoisolator circuit is introduced, similar to the PWM connection of the BMS to FBL ?