GENERATOR

oops sorry about that, I put one extra charge in there, it should read "speed charge" in other words, quick charge, the opposite of slow or trickle charge. 

Yes, In my opinion, that would be considered "quick charging" 

Correct, but with parallel batteries you always want to hook up the load or the charging source via the + of one battery and the - of the other.  This allows for equal charge/discharge between the two batteries.  If you only hook up to one battery, the resistance of the wires that create the parallel connection ensure the other battery will be discharged less and then charged less.

That can't be answered without knowing the capacity of your battery.
But- for a 105 amp hour battery, since most tow vehicles can't deliver over about 10 amps, then from 50% you are looking at about 6+ hours.
If it is the smaller 80 amp hour battery you are looking at about 5 hours.

 I have two paralleled 105 amp hour batteries and my tow vehicle with large wiring I installed puts about 13 amps into the batteries at idle- so in my case 50% down would take a whopping 9+ hours!

Which Is why I don't charge that way unless I'm going to be driving a lot-   on generator with the built in convertor it is putting around 35 amps into the batteries.  The call it a 50 amp charger but I've never seen it actually put out that much.

Don't forget charging requires you to put back more than taken out due to losses in the process.

We only dry camp. 3 days on a fully charged group 31 battery will work fine using the heater, water heater