I've adjusted many an RV 2 - 4 - 6 wheels, no different than your goose neck. Only way to do it is to keep adjusting up and spinning the wheel until it don't turn, then back off until the wheel turns with a little drag like the shoes are just touching, you can hear a little scraping. Don't know any other way to do it. The magnets grabbing and disc condition and how loose the arm is, is what determines how hard the brakes are applied from wheel to wheel.
Edit, I always tapped the wheel drum lightly with a hammer after adjusting, sometimes that will loosen them up a little more., kinda resets the mechanism.