I suppose one of the safest methods of securing a trailer, also would make it the most inconvenient for the owner. Remove the spare off the back and take both tires
and wheels off...store them in a locked garage. Use a jack to lower the trailer to the point, the trailer would need to be jacked back up to put the tires
back on. Now how many of us are actually going to do it
But like Jack says
Most thieves are opportunists, and usually go for the easy picks. Always try to make your property just a little more secure than your neighbors.
And carry good insurance!