6/3 NM-B (Non-Metallic) across the yard
Cable in conduit

Say what?  It's never prohibited to run a wire in a conduit. What's prohibited is running wires in too-darned-small conduit, which is a problem of the cheap.   Well, in fairness, it's easy to mis-guess with cable, because a flat cable needs the same space as a round cable of the wide dimension. (because they twist). 

For instance, with 1 cable in conduit, the conduit ID must be 138% of the widest dimension of the cable.  6/3UF is flat and 1.223" wide, so the conduit ID must be 1.680".  That puts you into 2" conduit. 

You need to use UF cable, because the cable you want to use, NM-B, is not rated for outdoor use, and lots of us have experience with NM-B cable failing outdoors. 

Why even put cable in conduit?

Because cables are the only kind I have ever heard of.  I didn't know there were other kinds of wiring. 

Meet THWN-2 wire.  It is actual wire, not cable.  Like what's inside an NM-B sheath, but with insulation that's up to the task, and wire markings to make it legal to use directly. Needless to say, this packs much nicer into conduit than balky cable, only needing a 3/4" conduit (though I recommend 1" to make the pulling easier). 

Also, it's a pleasure to pull, since each wire flexes independently. 

Downside: THWN-2 cannot be used outside a conduit.  So the entire route must be conduit.  

Upside: Shallower burial depth, only 18" cover isntead of 24" for cable.  Also, Rigid or IMC metal conduit only needs 6" cover - expensive, but the upside is you can trench it with a garden trowel. 

Upside: smaller wire. 50A only requires  #8 Copper THWN-2 instead of #6.  Further, #6 Aluminum THWN-2 is also acceptable, for some cost savings, but this requires a socket which is CO-ALR rated. The breaker lugs are already aluminum rated. 

Boxes at the pole

That's pretty easy. They already make a variety of outdoor boxes for mounting on poles, intended for RVs. Just use one of those.

You can get a simple one, that simply provides a receptacle.  

Or you can get an RV "sub-panel", that provides a 50A, 30A, and normal common outlets all in the same panel.    However, this will require installation of grounding rods on-site and a ground wire back to the panel must also be run. The ground wire deals with ground faults; the ground rod deals with lightning. 

Certain applications will also require GFCI protection on the 50A circuit.  If you need that, it needs to be a GFCI circuit breaker back in the main panel. Upside: the 120V socket doesn't need it now. 

You come into the bottom of the box with conduit.  Even if you use direct burial cable like UF, you must still use conduit for the rise from burial depth to the surface and on into the box.  Remember your 6 AWG UF cable will require 2" conduit. Make sure your box has a knockout that big!