If you have money to burn... then get 4 6v and be done with it. A good set of batteries like 4 Trojans are going to run you around $500. You can buy cheap batteries. I don't.
But if you don't have money to burn - and you don't want to waste money - you need to realistically assess your needs as a starting place and try and build off of that.
First. How much boondocking do you really expect to do? The RV parks are full of of rigs our size plugged in, but the boondocking sites we are usually alone or just a few others.
If you plan on just going to a NF campground for a couple of nights and you're not running alot of power hungry devices you could get by with a simple set of two 12v (more on that later)
Second - have you assessed your power needs? If you've never boon docked then you are only guessing. That's why I suggest a power monitor. What do you want to run when you are off grid? Are you going to be running TV, Satellite, Office Equipment, Microwave over long periods of time like we do? If so then 4 6v are a must. If not then perhaps you might want to wait.
How do you plan on recharging that bank? Do you have a generator? Are you going to install an inverter?
What do you plan as far as energy conservation? Are you going to install LED lights. (lighting is one of your biggest draws in the RV)
As Dave said - 2 6v batteries are going to full fill most peoples needs.
That said - to address your question about adding 2 now and two later. I know a bunch of people are going to jump in again and say never do it but that is the simplistic dumbed down answer that is fed to people by the battery industry or well meaning individuals giving the simple safe answer. The true answer is that IT DEPENDS... It depends on the age and condition of the battery. If a 12v battery is a year old, hasn't seen a lot of deep discharges, been well maintained and properly kept watered & charged you can most likely add a second battery and they will be just fine. It is true that if you add a new battery to a weaker one - the weaker one will pull down the newer... But if the original battery is in decent shape its not going to pull the new one down that much. I've had Cruising Sailboats with large (600ah) multiple battery banks for over 30 years, and I (and many friends) have done extensive upgrades and added batteries on many occasions with no problems, but I know how to take proper care of a bank of batteries. It depends...
That said - since you are new at this stuff, if you boondock a lot and really use your batteries, you are probably going to be rough on your first set of batteries as you learn - maybe not - but a good guess. Unless you learn about proper charging, temperature compensation, equalizing etc. So I'd venture to say that if you put 2 6v in now they are not going to be in the best of shape a year or two down the road if you use them a lot and you want to replace them if you put in 2 more much later. 4 6v batteries is a pretty large bank.
As for cables - most people make their own since each job is custom. You can get some off of the shelf I imagine. Sizing is not dependent on how many batteries you have - it's dependent on the loads being pulled out of the batteries.
If you are just adding a bit more reserve capacity to the RV but not adding any large power hungry devices the existing size cabling is fine for now. If you are putting in an inverter then the cable needs to be resized. Proper cable sizing is a whole post in itself.... It depends.
There's a ton of good info out there where you can learn about this stuff. So if you don't want to wast money my suggestion is start simple, assess your needs, learn your options, then make your plan and execute it.
Here is a link to the upgrade that I did to my current 5'vr. You can see some of the decisions, options, and sources of materials. The battery box I obtained from Fisheries Supply. Northern Arizona wind and sun is a good source for a lot of 12v items like welding cable, lugs, breakers etc.