Proper grounding and service wire choice for remote load center, RV pedestal
The answer is "electrician in trade since 1976". In 1976 a 3-wire groundless connection was legal. Certainly not today.

Even a 1976, a separate neutral and ground wire was an improvement. He made it sound like the AHJ would prohibit it, seems unlikely. 

Today, belt and suspenders is mandatory. All sub panels must have ground and neutral brought separately, with separation in the sub panel. And then, an outbuilding or other off site location needs a separate ground rod. 

And unless the ground rod tests to 25 ohms, it needs two ground rods. Unless you have the equipment, testing is more expensive than driving a second ground rod.

If you're in conduit, use THWN or XHHW. It's cheaper.

if the conduit run is continuous, don't choke cable down it, use individual THWN or XHHW conductors. Your neutral can be smaller and your ground can be much smaller.  

For instance I just priced 2/2/4/6 in XHHW and it was $120 for 100 feet.   I don't see any 2/2/2/4 URD or anything out there at that price. 

Mind you I got these prices by calling my local electrical supply house: trying to buy wiring on the Internet doesn't really work. Too many prices are sucker prices, or lowball but "gee, we're out of stock but thanks for joining my email list" prices, and then there's shipping to contend with. There is also junk imported from far-east places that use US wire sizes.  You don't want that. US big-box stores don't have any selection.  Proper electrical supply houses aren't online, these things have a very low price-to-weight ratio, so selling online is a fool's game.

Why URD is not a good fit for feeder

Feeder, service lateral and distribution are three separate and different things.  Your location is on your side of the meter, which makes yours feeder.  

Power companies follow a completely different and more liberal code.  They are allowed to use AA-1350 aluminum, it was designed for power transmission and distribution after all!  URD is intended for distribution (pole to pole) or service lateral (pole to demarcation point which is before the meter) and that is abundantly clear:


from product descriptions (which say that plainly, even if many don't understand the terms and so, disregard them)
product brochures (note the truly screwball ampacity ratings, and footnote that "by the way" these don't apply to NEC usage which must come from NEC's tables).  This is glossed over because product users are expected to know it. 


Now if you selected this cable because of these wild ampacity ratings like 120A for #4, that doesn't work at all. You need to use NEC rules for determining ampacity. For instance #4 Al has the same ampacity as #6 Cu, only good for a 60A subpanel.   

So why does groundless 2/2/2/4 URD exist? For service laterals for three-phase wye service, hot-hot-hot-neutral, either wye or wild-leg delta. Why does Home Depot sell it? Because it sells.  Given how bad their help is, I'm sure a lot of what they sell is misapplied, or wasted when the inspector catches it. 

Also, the cable does not have a ground wire.  People look at the yellow striped smaller wire and go "that's the ground because yellow is kinda like green". No, it's the neutral.  It's legal to remark #4 or larger wires, so it would need to be marked with green tape to make it a ground, and a hot conductor would need to be re-marked with white tape to be neutral.   I bet almost nobody does that.  

A fair bit of googling reveals many claims that URD is not legal inside any home.  The claims are that NEC doesn't mention URD so it's not a proper wiring method.  That does make sense.