My builder is about to break ground on our new home on a large rural piece of property that we own. There is a connected power pole with a 100 amp breaker box that is hooked up mainly to give the construction workers power for their tools. I also keep my large 5th wheel camper there for occasional stays. The camper has 120v, 50 amp service. During construction, I will be spending a lot of time there and want to provide power for it. The only place I can park it is 150-160 ft from the pole. Here is my plan. Please tell me if it is inadequate or incorrect in any way as I want to do it safely.

Purchase a 500 ft roll of 2 gauge THHN wire as well as a ground wire. I plan to bury the wire in PVC conduit and run it all the way 18" underground to a 50 amp RV receptacle near the camper. Is two gauge big enough to compensate for the voltage drop from the long run? What about the ground wire size. I've been told that 10 gauge would be OK but others told me to use 6. Still others told me to match it to the hot wire size. I can't imagine that I would need to go that big for a ground wire but I'm not an electrician. I even considered just running the conduit on the surface as a temporary installation but I'm not sure if that's legal even if only temporary? Any help, ideas or advice would be appreciated.

Signal13,
Temporary wiring in code is required to be protected... with construction going on around you I would support your intention to bury the conduit and conductors.
Based on 120V, 50 amps at 80% load, #4 AWG copper will give you a 4Volt or 3.3% drop. #2 AWG copper will give you a 2.5Volt or 2.1% drop, the recommended max drop is 3% although for temporary wiring not required.
The ground wire for a 120V, 50A outlet would be a #6 AWG copper. The NEC 250.122(B) requires the equipment grounding conductor to be increased proportionately with the increase of the ungrounded conductors... so if you use #4 AWG the ground would be #8 AWG, if you use #2 AWG then the ground would be #6 AWG.
When your house is built maybe you can reuse this feeder for a garage?

Great answer. Thank you very much!

Are you sure it's a 50A 120V? I have never seen one of these for an RV.
Typically it is 30A 120V (3-wire) or 50A 120/240V (4-wire).

Now Im beginning to question tranhat myself. Im going to have to double check. I didn't think anything in the camper ran on 240.

Geez.. thats going to be a little more expensive. Just measured the run today. Its actually closer to 175 ft to my proposed outlet location. Then I would be plugging a 30 ft 50 amp RV extension cord from the camper to the outlet. Does that change anything?

RV doesn't have to have a 220 appliance, with this hook up it can run two 120 circuits in RV

"Purchase a 500 ft roll of 2 gauge THHN wire as well as a ground wire. I plan to bury the wire in PVC conduit and run it all the way 18" underground to a 50 amp RV receptacle near the camper. Is two gauge big enough to compensate for the voltage drop from the long run?"

I just happenened to re-read this note and caught the reference to cable type "THHN". "THHN" cables cannot be used in a underground installation, regardless of installation inside of conduit or not. This type cable is for use in a dry location only.
Types "UF, THWN, THHN/THWN and THHN/THWN-2 can be used.

Because of the load, you must use the 60"C" column from NEC Table 310.15(B)(16) to select the correct cable saize. Then you do the voltage drop valculation,

I also questiion the 50 Amp service to the camper. NEC sets the load at 30 amps. Does your camper have a panelboard listed and rated at 50 Amps?

I've done some research. My camper runs on 120/240 which means I do need a 4 wire cable. My plan now is to attach a RV plug on one end and possibly hire someone to make the connection to the fuse box. Im not sure how to make the voltage drop calculations now. Do I use 120 or 240 as the voltage for the calculator? I have a contractor friend who has a few hundred ft of extra 6/4 copper wire that he will give me. Not sure what its called but it is of the outdoor variety. Any chance I can make that work?