I have the same set up as you (minus the NOCO) and want to share my experience with you regarding wiring the solar panel directly to the AUX battery. I have the USI-160 traxide system, not sure if you have that one or the SC80. The solenoid is designed to work with the charge coming from the MAIN and going to AUX, not from AUX to MAIN so it may not always join batteries when you put a charge on the AUX with the solar panel. I was really bummed when I checked my voltage meter and found only the AUX battery charging in my system, so I called and spoke with Tim at Traxide to get some clarity on the situation.Quick sketch in the pad. Main takeaways is there is a lot of redundancy in charging capability but the batteries are isolated via the traxide. Solar charge runs through the aux battery via traxide to keep primary topped off, essentially a reverse of the alternator. That was deliberate since the aux runs the fridge 24/7. NOCO, when used (rare at the moment) goes straight to each battery.
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I can't find my notes at the moment so don't remember the specific voltages required for the solenoid to join batteries, but it was high like 13.2 volts. So, if your main battery voltage is too low the solenoid won't join to allow the charge coming from the AUX to go to MAIN. The other thing was the solenoid timer will reset after 72 hours if the engine isn't started. So, if you're sitting in one spot for a week, the solenoid will disengage after 3 days and you'll have to start the vehicle to reactivate and reset the timer.
So, you may want to reconsider as I am, and wire the solar panel directly to the MAIN so you'll always have a charge going to the AUX battery through the solenoid from the MAIN.