I was running a 600w 35a Vector (with a 1000w inverter) at 35a to the trailer battery with the truck battery voltage holding at 13.8ish (needle holding pointing up) truck idling for over an hour.
With the losses, I think that means the alternator would have been doing more than the 35a the charger was doing?
The reason to do it this way is so your trailer battery gets the constant amps in bulk profile and to the 14s with the Vector, but would be stuck at 13.8v and tapering amps doing it just by jumper cables or 7-pin truck to trailer