To conclude this.
I called Cay Electronics, my local Mastervolt dealer/installer. They didn't have the Magic 24/12/20 DCDC charger but recommended a Victron 24/12/25 instead. I mention I have one of those but the wiring @ 10 awg worried me given 15A vs 25A. They indicated 10 awg is fine for 25A, it's a voltage drop problem vs a safety/fire risk. 10 awg at 25A would be between 5 and 10% voltage drop which they think is fine for a charger so they would change the fuses to 15A (24V side) and 25A (12V side). I'll do this and see how it goes.