Here ya go:
Get one of these at most any dealer with a camping supply store in it.
While this will not get you 50 amps of power from a 30 amp circuit, it will be a "must have" item in your RV for when you run into other CGs that only have 30 amp receptacles.
To do a proper and permanent fix on your lot, you'd need a position in the electrical panel the feeds your lot for a 50 amp breaker, you'll need that 50 amp breaker, 4 conductor cable of the right size (4 ga most likely - others will give you that or a link to a table with it -
here's one), a 50 amp RV receptacle that is mounted in an exterior grade box that can be mounted to a post and associated misc., connectors and such.
Jim