You don't need to hook up to the truck, a battery and jumper cables is actually preferred for testing as it eliminates the truck from the equation.
Are the small boxes circuit breakers?
or possibly relays?
I would go with wa8yxm's suggestion of making a really long ground wire. Hook one end of the wire to the negative of the battery. Hook the other end to the negative clip of your test light. Hook the positive of the battery to the trailer wiring. Then go pull bulbs and use the test light to check the sockets for power.
