Toyota not charging auxiliary battery properly

At work, we have a 2013 Toyota RAV4 that has been modified into a satellite uplink vehicle. An auxiliary battery has been added in parallel, along with a battery isolating solenoid, controlled by a Kussmaul 091-139-2-12 Auto Isolator. The issue is that the auxiliary battery is not receiving proper charging from the alternator. My question is where and how does this Toyota regulate and charge it's battery. Is there a conventional voltage regulator, or is it some sort of software-driven application? I can't seem to find a diagram & I'm trying to avoid having to deal with the dealership if possible. I know this is a vague & confusing question... :coffee: Thanks, Andy