The three lights that are out are on a common circuit. The curbside light is on a different circuit. As a quick start, pop the lens off all three of the failed lights, and remove and replace each bulb. They are subject to corrosion, and this may rub the corrosion off.
The fact that the whole string of three is out suggests a single problem.
1. There may be an interior switch for these lights, on the wall near the door. I'm sure you've made sure it is ON. (The lights may also have an individual switch on each fixture.)
2. A common problem is a bad ground. A quick test is turn on all the lights, pop off the lens of one of them, get a short piece of wire, touch one end of the wire to the shell of the bulb and the other end to a known-good ground, such as a screw protruding through the aluminum skin of the trailer, or the cut edge of the hole in the skin (which is ground). If the problem is a bad ground, this will light up the lights.
3. There is a possibility that the master switch is bad. Pull the switch out of the wall - this will expose the two wires that carry the power to the lights. Do not let these terminals touch the aluminum skin at the edges of the hole. As above, get a short piece of wire and touch the ends to the two switch terminals (and NOT to ground, which would blow a fuse).
Let us know what you find.
Bill
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