After a little research, I came to the conclusion that the 12v from the 7pin connection will not be sufficient for charging the trailer battery.
The 12v lead is only 10awg and was designed to trickle charge the trailer battery and won't do much to recharge a ~11v
This left me with 2 option:
1) Fishing 2x 2awg from the truck battery to the rear of the truck and finish it with a Anderson plug.
Now I will have full alternator amp. Upside is (one day) I can also use it to power a rear winch [emoji6]
2) This is the most common (boring) option chosen by most camper: buying a small genset to recharge the trailer battery.
Upside is no need for elaborated install. Just plug and charge.