Guys I don't get that one. You're down 100+ amp hours from full, start up E450 and maybe get 30-40 amps back to batteries at best. At least that is what mine is fused at 40 amps I think. How much time would it take to get that last slow 10-20% state of charge back into batteries? Maybe on a very long day driving.... But I can't see it fully charging batteries off alternator on a more local trip 1-3 hours from home say.
Nothing to do with group 29 vs. 27. Just heads up I don't think you can trust the alternator to completely charge the batteries unless you complete a very long day of driving.