What guage wire?

I would go with six ga for that run, though I agree 4ga is also an option.

The bigger the wire the better the wire the wire is bigger when the GA is small

For a 30' run the price difference should not be all that bad

Option 2: Go to the electrical supply and take their advice.

Note on options 2: If you pay the store, and it turns out they gave you bad advice.. Some might argue they owe you $$$$.. Where as here, Well, At most I owe you what you paid me ($0.00) so your Free is hereby cheerfully refunded :)

Two pole 50a breaker(240v) and #6 wire.

http://www.myrv.us/electric/Pg/50amp_Service.htm

The circuit breaker should be 50A two-pole (240V) breaker, unless you have a breaker at the receptacle in which case it could be larger if the wire going to the receptacle is sized adequately for the larger current. (You may wish to do this if, for instance, you foresee having some other electrical things besides the RV receptacle there.) I would suggest considering the installation of a campground-style power box, with a 50A, 30A, and 20A receptacle; this would generally only be wired with a 50A circuit anyhow.

Wire size is based on a bunch of factors, but boil down to two considerations. The first is ensuring that it doesn't get too hot, which in turn depends not only on the wire size but also on how and where it is installed. (Individual wires in open air have better heat dissipation than wires grouped in a conduit or cable, for instance.) The second main consideration is voltage drop to the load, which depends on the length of the wire as well as its size. I would guess 6 or 4 gauge would be used, but please, please, pretty please do not take my word for that. You'll need three wires plus ground (a total of four conductors).