How long to recharge RV battery while driving?

I have had that same question. I tested mine after I got back from a trip.

My 2 6-volt T-105's (225ah) were down to about 50 percent when we started driving down the road. I used my hand-held volt meter to test that percentage. I tested the rested batteries the next morning after a 10 hour drive. My volt meter tested them at 90 percent. Nothing but the parasitic draws while driving down the road. No fridge or TV. :)

Not scientific by any means but it satisfied my curiosity.