6 volt batteries are just half the voltage, not half the size. Size has nothing to do with voltage. Those little AA batteries for flashlight are small, but the same size could be an 18 volt batery.
Benefit of two 6 volt is that they run in what is called "series" not "parallel" meaning they one battery's current flows into the other battery and this doubles the voltage. Is like when you stack two D cell batteries in your flashlight and this makes 3 volts.
Parallel means that the two batteries are both feeding into your camper but are not connected other than at where they are connected to the camper.
When two 12 volt batteries share a connection then if one has a better charge (higher voltage) then the weaker battery will pull voltage from the stronger one to try to charge itself, or said in another way, the voltage will try to equalize between them, but all that happens is that the weak battery drains the strong battery. If the weak battery is 11 volts and the strong is 12 volts then at that moment you will see 11.5 volts, but the 11 volt battery will eventually deplete the strong battery down to 11 volts also so both will be 11 before long. 11 is considered a dead battery.
The 6 volt in series will double the voltage (add voltage of one to voltage of the other, 6+6=12). There is no parasite battery and one does not drain the other. You could have a 6 volt batt and a 5 volt batt to add up to 11 volts and you will have 11 until you drain the battery with your lights running, or TV or fan. But the 5 volt batt will never drain off the stronger 6 volt batt.
Good that you took this advice without even knowing why I guess.
You will not need a converter for boondocking. Likely your converter is fine. Dont mess with it. All it does it turn the Household current into battery-type current, DC current so that the DC power things in your camper (lights, water pump etc) will run off the household current rather than run down the battery. The converter almost always has a battery charging function built in.
You will need an inverter. Inverter takes DC battery power and makes it into AC household power. Most inverters are what is called modified sin wave. Sine is math and spelled sin, not like Adam and Eve sin, but sin as in sine wave.
A modified sin wave is square instead of nice and curved like ocean waves. Some devices will be harmed by a modified sin wave, but most things not. Things that use a power adapter like your phone charger, laptop plug will be fine. Try to power your items from the DC outlet instead of the inverter if you can.
1,000 watt inverter will be enough. This will be called 2,000/1,000 meaning it runs at 1,000 watts, but can peak out at 2,000 for a second of two. If you need to run power tools then you will need more wattage, but for TV this is fine.
Be aware that inverters use a lot of power. Running two TVs and charing all the kids **** will run batteries dead. You will not have remaining power for the heater, or fans, or lights or the water pump to flush.
Have the kids turn off phone apps that eat up battery on their phone. Kids are not savvy with electronics, that is a myth. Have them turn off the bluetooth, turn off the wifi and turn off cell if there is no signal. Those eat up battery and then they need to recharge off the camper more thus running down what little battery you have.
If you must have a boob tube then get on that runs off DC power, not AC which requires a wasteful inverter to turn DC into AC to run the TV which actually turns the AC back into DC anyway inside its workings.
Keep a battery lantern as you will probably have dead batteries at night.
Recharging off a generator will take a long time and you will need a stronger charger than is built into you camper converter. Dead batteries can take hours to recharge depending on how strong the charger is.
Having at least 200 watts of solar on the roof will help tremendously. With solar you should charge things early in the day when the batteries still have time to recharge before it gets dark.
With solar you actually are still running off the batteries. The solar charges the batteries and your power comes off the batteries. Electrically the power is all the same, whether battery or solar, but the solar is not providing enough current to run your big needs so the battery is supplying the current.
Think of filling a water bucket with two hoses. A big hose and a little straw size hose. Both are putting water in, but when you use a lot of water it is the big hose that is really contributing to your needs. The little hose represents your solar, and the big hose represents you battery charger that is powered by the generator or household current.
While driving your car alternator can also charge the battery, but it is also quite a small hose.
