How can I get 30 Amps @120V using several 12AWG and/or 14AWG cables?
Let's go over the ground rules and we'll build up to the solutions. 

Code is your friend.  Code can seem annoying when you're trying to do something, especially when you get fixated on one style of solution.  And many Code requirements seem rather silly, especially to someone with an EE background.  And newbies tend to think they can "outsmart" it.  But the more you know Code, the more sense it makes.  This isn't indoctrination, it's understanding the why's. 

The Authority Having Jurisdiction (AHJ) is the decider.  This is your local city inspector, the one who requires you pull a permit.  If you don't know much about Code, you may not know about the need to pull permits.  This means an electrical inspector will look it over, and this means you better be Code.  Cheating around the permitting process will get you in big trouble later. 

Redundant circuits disallowed.  With outdoor work, you cannot have 2 circuits going to the same place unless they are different somehow: voltage, switching, etc.  That means if both circuits from the main panel are 120V, you should put a switch on one of them. Nothing says you ever have to throw the switch.

Mixing disallowed. Each neutral must stay with its partner hot(s).  Grounds can mix, but only with other grounds from the same panel. This is relevant since you have circuits served out of different panels in close proximity.  You can't even mix their grounds.  

The RV site needs a grounding system of its own

This is true no matter what you do.  There are several ways to create a grounding system, but the usual method is to drive two 8' copper rods into the earth some distance apart.    This is needed because under some conditions, earth under your feet is not at the same voltage potential as earth 200 feet away.  This eliminates the possibility of being shocked while standing in a puddle holding a grounded thing. 

This local earth ground does not replace the equipment safety ground wire brought over with the conductors.  Ground needs to return enough current to assure a breaker trip in the case of a bolted hot-ground fault.  Dirt doesn't conduct well enough to do that.  

This is important because some people don't like installing a main service as now you need a grounding system.  Makes no difference, you need it anyway. 

Plan A (and revised) is hopeless

There are too many reasons to even discuss.  Some of them sneak up on you because it's unexplored territory. (a bit like aluminum wire seemed like a keen idea until...) For instance, what about overcurrent protection for the neutral wires?  Not an issue for a single neutral, so nobody thinks about it.  You'll just keep running into more and more.  What if the neutral between main and sub breaks? Dissimilar wire lengths. Dissimilar wire gauges.  Eddy current heating. On and on.  Given McCarthy evaluation there is no reason to deeply analyze this hopeless dead-end. 

Plan B: Do as every RV owner does, everywhere.

Get a cheater cable with 30TT socket to 5-15 or 5-20 plug, and and be careful with use of RV accessories not to overload the circuit.  Since a cheater is a removable cord and not part of fixed wiring, it's outside the permitting/inspection process. It's not Code, but doesn't seem to cause much trouble.  RVs have lots of other trouble with 22% of all RVers report being shocked, but this won't do that.

Since you mentioned this doesn't work out for you, I assume you really need the full 30A at 120V - 3600 watts or 3600 VA (3.6 KVA). 

Now things get fun.

Plan C: Take the red pill: 3.6 KVA

We use any one cable to send 240V @ 15A  (3600VA) from the house.  At the RV site, we use a transformer to turn that into 120V @ 30A. (also 3600VA).   The transformer absolutely needs several properties. 


Outdoor rated
240V primary (240-480 is a common type, that's fine).  
120V secondary (120-240 is fine if it the label says it can be jumpered parallel; most can).  
3.6 KVA or larger (5 KVA is common). 


These are commonly seen on eBay or Craigslist for $100.   Of course a new one is likely to be quieter.  Shop around, prices vary a lot. 

Since the long haul is 15A, you can use any cable. I'd use the 12/3 simply because it goes to the right place.  In that case, neutral would be unused. 

Now we go down the rabbit hole.  Read my answer here, particularly plans 3-4, about how using a transformer makes it a main service.  That's why you need a local grounding system, so you can have the one neutral-ground bond that every main service must have. 

The post-transformer side is freakishly simple.  The transformer secondary goes directly to hot and neutral on the TT30.  It doesn't matter which one. However it does matter that the "neutral" side is bonded to your locally derived ground, and the ground pin on the TT30.  This is the one place on this side of the transformer where a neutral-ground bond occurs.  As ThreePhaseEel mentions, this is allowed under 240.21(C)(1).  All of the overcurrent protection is provided by the 15A breaker back at the house (ah, the long walk). 

Plan C-2: A panel, and a little more load: 4.8 KVA

Same as plan C, but use a 12AWG circuit, and breaker it for 20A. Since this is 4800VA, you would definitely need a 5K VA transformer. The transformer secondary goes to a main panel as discussed, and neutral and ground are locally derived, and bonded in the panel (the one place).  It has a 30A breaker going to the TT30.  If the RV trips the breaker, it'd be more likely to trip here.  

You could also put 1 or 2 more circuits in there to other things, but the combined load could trip the breaker back at the house. 

Plan C-D: Get ready

Same as plan C-2, except you use a 10KVA transformer, which definitely has 240x480 on the primary and 120/240 wiring on the secondary. This will allow you to later bump to plan D with only one more transformer. 

Plan D: Now you're playing with power

This plan delivers 9.6 KVA over 12/2 cable, allowing you to power a larger RV which takes 240VAC/30A (7200VA).  It uses particularly hazardous voltage, so you'll want professional help for this. 

In this plan, you use two 10 KVA transformers: The one I mentioned in plan C-D, and a similar transformer at the house.  

The transformer at the house will be back-fed (which is fine). Jumper both transformer primaries for 480V and connect them back to back.  The power will be transmitted over the long wires at 480V and half the amps.  That means you can deliver 240V/40A (9.6 KVA) on 12AWG wire - or if you prefer, you could jumper the transformer to serve 120V/80A.  This would need to go into a "main" panel again with locally derived ground and neutral.  

This should scare you a little, but the 480V will only be present in 2 places, and only inside transformer enclosures. 

Plan D-2: power any RV

This requires the rarer 120/240V to 600V transformers, at 12 KVA or larger size, and the 20A circuit.  The house side gets breakered 50A.  The RV side is now allowed a full 50A of 120/240v.  You can now install one of those RV multi-stands that supports 240V/50A, 240V/30A, 120/30A and 120/20A (one at a time obviously). 

By the way, the absolute limit of insanity here is 21 KVA over the 12/3 cable, by using a 3-phase converter and a transformer that sends the power as 20A/600V 3-phase "delta".  That's enough for 6 of your RVs.