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Without more info it is hard to tell.
Make/model of truck?
Who wired the connector on the back of truck? Factory or paid installer?
What was the truck wired to pull? Some trucks are wired so that the center pin is for the reverse lights, and others for 12v power.
I did come up with a fun way to solve a problem i used to have related to this and due to the new truck. On my old truck (2005 Dodge) I had a small (45W) solar panel I would hook to the batteries when in storage for more than a few weeks. Had to do this after replacing the batteries 3 times on the Costco warranty - they'd fail over the winter when I used the truck much less.
BUT - my truck is parked in in the driveway in front of the camper. And on the new truck my 12V always line is live all the time. I now leave the truck connected to the umbilical cord from the camper whenever I'm in storage. Keeps the truck charged from the camper....both via my solar on the camper and via the shore power. Just figured this out and it works as I can look at voltage on the truck battery with a meter and compare to live with the voltage on my Victron BMS via bluetooth.
It's the little things that make me giggle a little bit
After opening up the fuse box and checking the owners manual I found the location of ?power to trailer 12v? I noticed a large relay was missing in this location and after talking with a guy at the auto part store he informed me that a lot of the newer F150s are sold with a relay in a baggy in the glove box that you put in if you?re actually going to be using the 7pin. But he couldn?t really explain why they don?t just install it?
Anyways mine was not in the glove box so he got me one for around $17 (plus had a coupon and a $5 reward so it was like $10) and after installing the relay the 7pin had 12v instantly.
Seems silly that it?s not just installed but it makes me wonder if it somehow drains the battery or something