I use locks on everything.
For the 5th wheel pins I use these:
http://www.etrailer.com/Locks/Reese/RP58093.html
This on the receiver:
http://www.etrailer.com/Locks/DeadBolt/RH5-XL.html
And this on the 5th wheel release handle and trailer coupler:
http://www.masterlocks.com/product/3...-steel-padlock
I also lock my tailgate.
When pulling bumper pulls I run the electric brake break away cable through the padlock, keeps it off the ground, away from the WD bars, and in line with the hitch.
I've had people try to sabotage me. The locks obviously deterred them a bit, but they did pull the pin on the break away cable and released the latch on my tailgate (that's why I now lock that as well). I was doing my walk around after a stop, noticed the break away cable pin was pulled. Went to reinstall it, put my hand on the tailgate and it fell open. If I had started down the road with the tailgate in that condition, it would have fallen open and possibly damaged the trailer. I lock everything and check/double check everything.