Off topic a bit but I already have the Dual and Smartpass installed. It's brilliant. No you don't need anything like a 100amp alternator. It takes 12v and ramps it up to 14.5 until your battery is completely full, then it backs off to 13.6 v I have been messing around with one all day today to see if it's best to put one in my trailer. It replaces lost charge in your batteries far quicker and more safely than a simple split charge. If you have only 12v going to your leisure battery at best it will only ever be about 80% charged. When your alternator ramps down after the cranking battery is refilled, any split charge is going to get a reduced input. With the Dual even on its own, that power gets boosted in a stepped mode until you reach capacity without boiling anything. My Optima has thrived on the CTEK. I can go to it after a week of it being parked and the Victron is still showing as 100% charged and around 14 v float charge.
Your alternator can be 300 amps if you want, but it won't ever supply that unless it's demanded. And there's nothing on a vehicle save a winch that'll ever try to pull that much. You're not going to be sending 120 amps to your leisure battery. The issue is when you park up at night and run all of your kit like your inverter and microwave etc, there's a huge amount of charging to be done the next day. With 6mm cable and standard alternator output, you might drive for 8 hours and never make that all up. You stop again and again and by the end of the week the battery is at 11 volts. The more residual charge that is left in the battery the next morning, the easier it is to charge back up again. Getting to 80% is easy, it's getting that last 20 % in that requires the woomph. They're not cheap though.
OK back in the room with Stan.