Never.
However I've carried a 14" long grade 70 transport chain and one of those padlocks advertised as bullet proof in three different 5th wheel trailers since '84. It wraps around the king pin and pad locked.
I've never seen any reports of a 5th wheel rv trailer being stolen where we go or where we live.
Now flatbed trailers just about every day in local news someones trailer full of some type of equipment gets stolen but most of them are bumper pull trailers.