I would recommend the Progressive Dynamics
PD9245c 45 amp converter to most closely match what he has now and would probably be a perfect fit.
If you need a more powerful converter, the
PD9260c 60 amp would be the next step up.
Both of these converters have the "Charge Wizard" built in to them and will keep your batteries charged up and maintained with no water loss and turn you converter into a "smart charger". We have had this type of converter in our rig for many years and leave it plugged in 24/7 for long periods of time (5-6 months) with no problems at all.
Barney