12v trailer plug

[Alpha88]

Hello, I have a 2018 gmc 2500 sle duramax. I have a quick lift loader in the bed. It's set up like a dump trailer with its own battery and electric over hydrolic pump. I tried today to make a 2 wire harness from a 7pin trailer plug to the "red and white" posts to go to this auxiliary battery to maintain it when I m driving around. I m not getting any power to the positive wire, I m also not getting a message on the dash when running "trailer connected/disconnected" when u plug it in and unplug it. I found the 30amp fuse under the hood, it has continuity through it, but engine running doesn't seam to have any power at the fuse itself.  Any ideas? Should this wire have battery voltage all the time with key on? Or is it looking for something to know there is a trailer connected? 

Thanks