Tongue coupler lock!... Also wrap safety chains around tongue and lock them...
IF wheels have holes in them, a cable lock through wheel through springs will help slow a thief.
Other wise If I want it, I going to take it no matter what you do....
Most thefts are spur of moment.... If you make it difficult they will go find one that is easier to steal.
I would be more afraid of the thief with the power tool scoping out trailer in my yard and studying when people are home or not... In other words a planned theft.
Dale