The problem I always run into was the truck system always see's the status of the truck start battery which is only inches away from the alternator and shuts down any high current charging for the additional battery 20-feet away from it...
I doubt if the truck alternator even knows it has a another battery in the circuit with the truck start battery so close to the alternator...
What the Emergency Vehicles uses is a second alternator system for all of their addition batteries and electronic systems etc...
I too am complicating doing something similar for charging a second battery bank mounted in the truck bed for my emergency radio operations and it scares me I might blow up my truck alternator setup... That's another golden rule to not do anything to your truck system as this is the only way you can get back home haha...
Roy Ken