Here are other alternatives to the inlet suggested by Bob:
15 Amp™ | Marinco
I just searched for the part number is Amazon.com and they are both available for less than $20 each.
I still recommend that you protect the circuit with a breaker in the trailer. It should be a dual pole, 15 Amp breaker. This follows the current safety practice for RVs. The reason for a double pole breaker is that you want it to cut both hot and neutral on the incoming power when the breaker trips off.
Find a small surface mount or recessed circuit breaker box and get a breaker for it. You may also use a marine circuit breaker panel with a 15 Amp double pole breaker.
Use stranded wire of the correct gauge. Do not use solid conductor wire. The vibration will eventually fatigue and break the conductor.