50 amp wiring for an RV receptacle
OK, a bunch of stuff to respond to:

Wire size

There are really two considerations when picking wire size:


Safety. You do not want the wire to overheat and catch fire. 6 gauge is fine for 50 amps, so you're good on this front.
Voltage drop. Long wires cause the voltage to drop. Upsizing the wire will counteract this. Typically you want to keep the voltage drop under 3% or so, although it somewhat depends on whether the voltage coming into your house is on the high or low end, and also what you're using the circuit for. 6 gauge wire for 500 feet @ 50 amps will have more than 8% drop, which is almost certainly unacceptable. If you want to stick close to 3% you will need 2 gauge at a minimum.


Cable Choice

Your RV hookup is probably 240V with a neutral, which requires 3 conductors + ground (not 2 conductors like a standard indoor 120V outlet).

Also, using "indoor" Romex (NM-B) is not appropriate for an outdoor installation, even if it's in conduit. You will need to use something outdoor rated, either a UF cable or individual outdoor-rated conductors.

Finally, 500 feet is a very long run. Make sure you plan carefully. Even simply buying cable that long will probably require getting a whole spool. And pulling the cable through conduit is a whole other challenge.

At this length you might consider using aluminum cable, which is somewhat cheaper for large sizes, although it definitely complicates the installation. I don't know much about aluminum wiring but I believe it requires special connections and the wire amp ratings are different.

Since you are already going to be using an electrician, I think you should ask him/her what they recommend.