I just use a Master brand locking pin. Maybe not as good as a full coupler lock, but hopefully will slow them down or move them on to an easier target. If someone wants it bad enough, they'll get it no matter what you use; but in a storage lot or CG there are generally much nicer targets than ours. The nice thing about the pin is that it is also locked while you are in transit, lest someone be planning a parking lot swap out.
2011 Jayco X19H (purchased 2015)
2008 Jayco 1007 PUP (purchased new, traded for the X19)
2012 Chev Traverse LT with TP