Whether or not the alternator is going to charge your house batteries is a matter of time/distance, not battery size. Any alternator on a Ford or Chevy van cutaway chassis is going to have the capacity to do the job, after restoring the charge from a startup, if you are running long enough. Other loads are insignificant compared to 100-160 amp alternator outputs.
I don't expect the alternator to recharge my house batteries after sitting for months, because my campsite is 15 miles from my house, barely enough run to take care of the starting battery. So I recharge, after storage, in my driveway before a trip. But most RVers travel a lot farther?