Why "12V operation" is never desirable in this configuration:
If your "genuine Solar" is performing well, and your Controller has determined that your batteries and loads need "Bulk" or "Boost" charging, the the Controller attempts to keep the battery charging Voltage fairly high: 14.2-14.4 Volts.
But, you have connected the Trailer's +12VDC power distribution to the Tow Vehicle, via the Bargman. The Tow Vehicle has probably "decided" the alternator in a way which yields 13.9V (and maybe even less). Your Controller's attempt to run at "Bulk Charging Voltage" leaks power into the Tow Vehicle - and the "thirsty" batteries get much less.
Whenever the Solar conditions aren't great, you should flick the switch and run in 24V mode. And, if Solar conditions are great, you should probably leave the Bargman "Trailer Battery Charge" wire isolated, disconnected from the Trailer 24V system. ("12V mode" in the Tow Vehicle = 12V with other Trailers, but nothing in a properly configured "24V" Trailer charging.)
