Proper amount of solar panels needed to charge batteries
I'm a little concerned with sizing here, but your math seems pretty solid.  1200W is 1200W, and the voltage of the battery doesn't change that. 

So yeah, if you want to charge a 7200WH battery pack in 6 hours, that is indeed 1200W.  You're not going to get that out of 600W of panel, unless you wait twice as long.  

Of course, you won't get 1200W out of a 1200W panel set except when it's square-on with the sun.