Mark - your system is exactly the opposite of what I have in my 101.
Truck system is 24v - starter, lights etc - with the same alternator as you have for your secondary 24v system.
I also have a second 65amp 12v bosch alternator running house systems such as aux 12v battery which runs my fridge, camp lights and ignition as it is now 12v.
The 101 (like your truck) is well earthed to stop electrical noise to the engine, gearbox, chassis, body etc are all well earthed together with substantial earthing straps. Whether 12v or 24v I just earth the gear nearby and and have never had an issue.
Good luck with it.
Garry
