Charging AGMs with truck alternator?

What I am thinking is adding 2-4 batteries in the bed connected to a charging circuit from my second alternator, I will run as large of wire as needed. I would install an isolation relay like one of these.

https://www.bluesea.com/products/7601/m-Series__Automatic_Charging_Relay_-_12_24V_DC_65A

This will power a 3000 watt inverter on my commercial canopy for work and add battery capacity for the truck camper when I use it. So I want to set it up with an umbilical cord to plug into both.