Usually on the firewall, because it connects to an e-start dash switch, power from ignition switch circuit and chassis battery. That's all true for a gasser, but yours being a diesel could change it slightly.
You also may have a Trombetta instead of a BIM as original equipment.
Has anyone located and changed the BIM to a BIM/225 as part of the lithium battery conversion?
Historically, the BIM-160 is located in the battery compartment under the steps on the Omni / Magnitudes. Usually on the right hand-side..... but it depends on where the guy felt like installing that day.
I used half of the front passenger side basement locker and put two 200Ah LiFePO4 and most of my Victron gear in there. Used old battery locker to mount my Viair compressor and for the dc/dc charger. Viair piped to quick connect in hydraulics locker.
Sorry, we're traveling for 4 months. The dc/dc is in the old locker fed from the OEM BIRD (bi-directional isolation relay) only about 4 feet of #6 to the Victron Lynx Distributor. The Lynx and batteries are basically backed up against the battery locker.