You have 2 completely different questions in two completely different scopes. Don't confuse them. You must consider and answer each individually, and use the higher of the two answers.
Given the receptacle you want to use, which defines the breaker size, what is the minimum wire size you must use to keep from cooking the wire? A 30TT or 14-30 requires a 30A breaker, and the answer is "10 AWG wire".
Given the current you realistically expect to draw, actually, how much voltage drop do you feel will be acceptable to you? This is more complicated, starting with the "current draw, actually". It depends dramatically on what appliances are installed in your RV, which appliances you actually do use, and how much current those draw. Which you need to survey.
Remember -- having a 30A plug does not mean the RV draws 30A. It means that is the standard socket at RV parks, because some other RVs draw 30A.
Continuing part 2, you need to figure out the voltage drop you find acceptable under your practical load(s). I for one don't have a problem with a 5-6% drop, but then, I don't feed my family by upselling you into thicker wire. Those who do, say 3% is max. 3% is not mandatory, it boils down to what you find acceptable. The voltage drop cannot damage your wire, the required breaker in step 1 guarantees that.
As an example, let's say your practical load is 10A most of the time and you'd like to have a 3% drop then; or 20A in extreme circumstances and you don't mind a 6% drop then.
Plug those into a voltage-drop calculator, and there you go. (only consider the distance from the last subpanel to the RV).
Now, take the largest (smallest numerically) in step 1 (10AWG) and step 2 (in my example 12AWG). You must use at least 10AWG.
If you did the numbers again with 3% at 24A (the highest you are likely to draw continuously), I get 10 AWG.
If you want to for thicker wire nonetheless, that's between you and your wallet.
Now consider the new AA-8000 aluminum alloy wire. The ooga-booga about it never applied to AA-8000, nor to large conductors. At those large sizes with lots of metal content, it is far cheaper, and bonds better to large lugs, which are made of aluminum.
Starting at 6 AWG Cu, I start pricing it both ways - you need to run the numbers and typically go up a wire size i.e. 4 AWG Al. For the cost, you could even upsize the AL and reduce your voltage drop further.
If you upsize the feed to a subpanel, you can upsize the supply breaker. If you upsize the feed to a receptacle, you must use the breaker the receptacle requires.
If there are any problems, they'll appear at the terminations first.
So as you work on that system, just keep an eye peeled for any thermal damage at terminations. Discolored wire, melted insulation, arcing, anything like that.
Truth be told, 14AWG wire is allowed for 20A - just look at the table formerly known as NEC 310.16... but this presumes 75C terminations (e.g. the receptacle will not scald your child's finger if he touches the outside while a wire is cooking away at 75C on a screw) and a bunch of other favorable factors. So there is a statutory rule elsewhere in NEC that clamps 14AWG ampacity at 15.
Best Answer
12/2 may require a 20A breaker
Note ThreePhaseEel's comment, where he notes certain motors require a 25A or 30A breaker (NEC 430.52) yet are permitted to use 12AWG wire (430.22 notably 430.22E), if so, this may have been legal after all. I would, at least, upgrade to 10AWG until it leaves the house. I care if it burns down your house. I don't care if it burns down your yard.
If the amp draw is well under 20A, then go ahead and fit a 20A breaker and now you're code legal in any case. And due to a happy coincidence of the numbers, your wiring run is almost exactly 1 ohm of resistance. That means the voltage drop equals the amps. It's a lot easier to measure voltage drop than amps: measure the voltage at the panel, measure the voltage at the pump, subtract. The pump must be running while you're measuring.
Yes, you can run 20A on 12AWG for unlimited length. Your load might not like the voltage drop, though. That's why I suspect the actual amperage is well under 20A.
Don't use a lightbulb for a heater. Use a heater.
Light bulbs burn out and you won't know when they do. Just go to McMaster-Carr, Grainger, or other industrial supply and buy a heater module. For instance they have a strip heater, 240V, 3 inch wide, 120 watt for $19. Any common strip heater will do fine.
Or even better, they also sell pipe wrap heating, which you can then wrap with insulation. Could take well under 100W!
There's no way to string 120V out there
The pump has 240V hot-hot-ground 3-wire electrical service out there. Neutral is not ground so there is no way to obtain a neutral to get 120V service. No big deal since you can get 240V heaters.
If you must do this thing with incandescent light bulbs, get 3 bulb bases and wire them in series to 240V. Then put 3 identical bulbs in there. Each bulb will see 80 volts (1/3 of 240v). Since that's 2/3 of its normal voltage, it'll run 4/9 (2/3x2/3) of its normal power. The bulbs will last a great deal longer, but when one fails, they all go dark.