I know a lot of people ask this question, and while looking in another forum (RVNet) I found this. So far, it is the most through explanation I have found.
I hope this isnt illegal, but since I figure we are all here to help one another, it cant hurt to re-post someone else's very good info.
This was posted originally by smkettner in the RVNet Forum.
Four GC batteries is about 450 amp hours. Down 50% (12.0 to 12.2 volts) will need 225 amp hours to get full. Actually closer to 300 with efficiency losses. The typical charge wire to the trailer will give about 10 amps regardless of the alternator size. Possibly a little more at the start and definitely less near full charge. Even if you figure 10 amps it will take 30 hours of driving to get up close to full charge.
Now you can upgrade the alternator, wires etc. Or wire in an inverter to drive a battery charger etc. And end up reengineering the whole thing. But ultimately you will be better off with a quiet generator and a modern three stage converter. You should be able to get set up for less than $1,000. The truck will still provide some power when traveling but the generator will keep you charged up while stationary.
If you run the batteries to 10.5 volts with any regularity they will loose capacity quickly and need replacement.