You can buy square link 3/8 chain that is damm near impossible to cut with bolt cutters and the biggest baddest master lock or two u can find. Will do the trick....lock the rims up
Ive tested trying to cut things myself to see how difficult it is. Most chains or cables will fall to a set of $40 lowes bolt cutters. Cheap locks can be tko`ed with a couple whacks of a hammer. Tounge locks are mostly crap and don't forget that even if u buy a bad a$$ one. Most couplers unbolt off the trailer with a half dozen bolts or less, so it'd be an easy swap to a coupler w/o a lock on it.