Actually he has an on-board three stage charger. Using shore power or the generator will charge the batteries much faster than the engine alternator.
If the house batteries are very very dead, then typically the converter will not begin charging the batteries until they get up to a nominal voltage.
Hence my suggestion to take a drive, the engine alternator doesn't care and is up to about 160 amp output, which is more than than the power supply in the converter puts out.
That can get the initial charge voltage up to where the converter could then finish the job.