One confusion when 6v batteries enter the fray is to convert to appropriate units for power and energy. Multiply Amp Hours times the voltage of the battery to get watt hours. That way you can compare the energy between batteries no matter their voltage. That also makes it easier to figure in the effect of appliance loads and solar systems if you want.
You'll find that you get about 22 watt hours per pound of lead acid battery +/- 10% or so depending upon current draw, age, temperature, cycle variations and other factors.
The reason for the 20 hour rate for RV type batteries is to get somewhat close to a typical RV duty cycle. There is also, for instance, a 5 hour battery rating that is used for batteries intended for commercial use where they are typically charged after every 5 hours of use. A typical group 27 RV battery with 100 AH (1200 watt hours) at the 20 hour rating means that it provided a steady 5 amps over 20 hours.
Another common energy capacity rating for batteries is reserve minutes. This is how long the battery can produce 25 amps in minutes. That can also be converted to a watt hours energy measure. You will find that ratings determined with higher currents will have significantly less energy capacity from the same battery.
The reason why there are many capacity measures is due to the factor the Ben et al are making fun of. The current you draw from the battery and how you draw it can make a significant difference in how much you can get from it. Take note also that, when people ridicule well established ideas, it indicates that there are factors at play that stray far afield from those needed for effective and useful decisions. Take care.
For longest battery life, you should size your batteries so that you run them down at least 15% to no more than 50% of their rated capacity each use. This means the battery voltage in a 12v
bank should go down to at least 12.4v but no less than 12.0v after they have been resting for half an hour with no loads or charging before you recharge them after your campout. (this is the answer to your question about how much charge to use)
Also, for long life, recharge promptly and properly. If leaving unused for more than a couple of weeks, use a sulfation inhibition charge maintainer on the batteries. The PD+Chargewizard or WFCO converters are very good at this.
Watts is amps times volts and is the proper measure of power. Solar panels are rated in the watts they provide at peak output and many appliances by the power they consume. Converting from amps to watts makes it easier to compare power draw situations.
Energy is power times time so it is amps times volts times hours. Your home electric bill is probably about ten cents per kilowatt hour. That is about 1 typical RV battery's available energy storage.
The constant duty at about double the 20 hour current to watch a movie in the evening has surprised several folks I know when it comes to battery drain. Intermittent use and lower currents can dramatically extend your off grid time, more so than simple arithmetic would indicate.