For anyone interested, I installed the dual battery system into my Laredo this weekend. Pic's below, but using set up similar to what John on this forum showed me.
Step1
200amp red cable from main battery under driver seat to a 140amp circuit breaker mounted at rear of main battery compartment.
Step2
200amp red cable from circuit breaker under carpet, under door sill trim under rear passenger seat mount to a Piranha 140amp solenoid.
Piranha mounted under swing flap behind rear passenger seat and earthed to rear tyre storage skirt mount using existing screw.
Step3
200amp red cable from Piranha to Battery box strapped down to rear seat mounts.
Step 4
200amp black cable from Battery box to large earth connect using rear seat mount bolt.
Step 5
Install SAAS 12v guage on RHS driver windscreen pillar, earthed to body near driver door hinge.
Run red cable (I used 20amp) from 12v guage down side of dash, then under door sill trims all the way to the 2nd battery. I included a small glass in line fuse near the 2nd battery.
Two hard part:
1 - make the decision to put 2 screws to mount to SAAS 12v guage in the pillar trim on RHS of windscreen. Figured I could always replace the trim if needed.
2 - running a 200amp cable under the carpet, then sill trim, then under rear seat. While not a great distance, it tested patience and self control.
All seems to be working so far.