Two words: Fish Tape.
I run a wired rear camera to my trailer.
On mine the yellow wire was the critical one. I am still going slowly with the manufacturer to make sure I use the correct wires, they only answer one question at a time, so it is taking over a week….
I put camera on the back, drilled a hole for the wire, ran the wire inside (not under), spliced in new wire to make it longer.
I used the fish tape to get the wire through the wall and out to the tongue of the trailer.
Fish tape is a flat flexible-in-one-plane metal that can be pushed and forced through small holes and poked out easily a hole far away - fishing.
It is a great tool to have.
I then used a two pole plug like this one
https://www.amazon.com/Femitu-Gauge-Quick-Disconnect-Harness/dp/B006ZOGYUU?psc=1&SubscriptionId=AKIAILSHYYTFIVPWUY6Q&tag=duckduckgo-d-20&linkCode=xm2&camp=2025&creative=165953&creativeASIN=B006ZOGYUU
to be able to connect the trailer and truck's camera wires together. But you may need a 4 pole wire (4 wires in one plug) for some cameras.
I then used an A-B switch to select either truck rear camera or trailer rear camera so only one shows on the screen. On mine I ran the yellow wires to the switch.
It will be complicated wiring it all up unless your camera only uses two wires. Mine 4.