First thing is to make sure your truck socket battery lead is operational.
Some trucks must have a fuse installed in order for the Trailer battery
connection to be hot at the truck rear bumper socket.
One way to tell this is with a meter.
Here is a photo of the typical 7 pin socket found on the back end of many
Notes- the notch is at the TOP in this photo.
The left side is looking at the truck socket with the door held open.
The right side is looking at the BACK wire side of the socket.
You're just interested in the left photo.
You should read about 15 volts between the 1 o-clock battery pin and
the 7 o-clock ground pins with the truck engine running.
It's possible your truck battery to RV socket fuse is bad or missing.
I believe it's found in the fuse block under the hood but it's been 7 years
since I had to install mine so I'm not exactly sure.....
Check this first! Maybe it will solve your problem.