Since your trailer has 50A service, you'd be best off using a 50A outlet. The problem is running wire to it. If you installed 1/2" conduit, the maximum number of #8 wires you should run in it is 3, one less than you need for a 50A outlet. 3/4" conduit will easily take the four #8 wires needed for the 50A outlet. If you're limited by the conduit size and don't want to replace it, Use #10 wire with a 30A outlet and use an adapter for the rig.
I see no problems running 50A wiring and using a 20A breaker. You can always easily change the breaker at a later date if necessary. You should also put a label in the outside box indicating that the outlet is on a 20A breaker.