The later model Rangers have a current shunt on the battery negative. All negative connections must be on the chassis side of this, including the main vehicle chassis earth. This is so the so called smart alternator can adjust it's output according to load.
If you put a large load (eg 2nd battery charging) on the wrong side of the shunt, the vehicle computer won't adjust the alternator output to compensate, so it will run the start battery down. They only charge the start battery to about 85% anyway, by design.
I have a 2020 Ranger, 140aH AGM battery and Redarc BCDC in the tray, earthed to chassis behind the cab. Never had a problem in this vehicle or the identical setup in my previous BT50 (non-smart alternator in the older Mazda).
Thank you. Helpful and detailed explanations. I have not encountered a similar problem, but for the future I will already know. The main thing is not to accidentally forget.