If you really want to protect yourself from someone cutting the sides of the trailer open, line the inside with some old car leaf springs, spaced I would say every 4-5" apart. Nothing ruins a cutting tool faster than a spring.
Remember thieves are usually lazy opportunists, make it a little bit hard, or even appear to be hard, and they will be more likely to leave you alone.
+1 for medical waste, septic cleanout, or some other such labels on your trailer to dissuade folks from invading.
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