Re: How much driving to charge batteries?
The rule of thumb is that you do not want to discharge your house batteries more than 50%. This is for optimum battery life.
To your question:
You have to think in terms of amps. Say your two house batteries have a capacity of 200 amp-hours. Discharging them to 25% would mean you drained 150 amps out of the batteries and that is what you must replace. Say you have a 70amp alternator ( meaning it can put out 70 amps every hour). Lets assume it takes 20 amps to run your van and its accessories. Engine, heater blower, headlights, radio, etc. That leaves 50 amps (70-20) that can be used to charge your house batteries. So AS A MINIMUM that would require 3 hours of charging. 150 amps to replace with an alternator supplying 50 amps every hour (150/50=3). It would in reality take longer because as the batteries get closer to being full the rate of inflow of amps will decrease.
Bigger alternator....shorter time. Smaller house batteries....shorter time. etc.
Len & Joanne
The Green TARDIS