The yellow thing is a 30 amp male-15/20 amp female adapter, similar to the one Tom showed above. That's just in case someone else (like a tent camper) is using the 20 amp outlet on the pedestal for a coffee maker, or something, or if the 20 amp outlet is out of service.
I've heard that RV pedestals have been known to be wired incorrectly, so I'll also carry the green outlet checker that lights LEDs warning if the line and neutral wires are crossed, or if the ground is open, or a few other problems. The red thing is a Harbor Fright non-contact power checker that should only light up when near the line, not the neutral or ground. It should also alarm if the trailer frame is energized for any reason.
If anything looks wrong, I won't connect. I trust my wiring, but not some RV camp's.
Tom