Well you have a few options. #1 do you have a test meter of some kind or even a 12 volt test light to see if you are getting at the plug? If so see if you are getting any current to any of the blades in the socket.
Are you dealing with a 4 wire flat pug or a 7 bladed round plug
Check your ground. Just take a standard automotive jumper cable and connect the black wire to a bare metal spot on the truck and on the trailer. Do the lights work now?
Get out the owners manual or look it up online and then check any fuse labeled "trailer" and replace any that are blown.
crawl under the back of the truck and see if the socket is even wired up.
Give us the year make an dmodel of the truck and we can help find the fuses for you.
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2001 Suburban 4x4. 6.0L, 4.10 3/4 ton **** 2005 Jayco Jay Flight 27BH **** 1986 Coleman Columbia Popup