You can easily install a second alternator in a Promaster. It's done all the time. I have the Volta system in my van. Volta supplies everything: alternator, dc-dc charger, mppt charger, inverter charger, battery pack, harnesses and cables, etc. It's one big integrated 48V system. Of course there's also a converter to bring it down to 12V for all your appliances in the van. It's a one button system that provides no visibility into what's going on which I hate, but it's very reliable and very powerful which I like. I was told by a Volta engineer that the reason it doesn't start charging until the RPMs get above 1300 had something to do with vehicle emissions. It doesn't really matter though as there's a button to put the van in high idle if you want. The second alternator also charges the battery pack from zero to 13.5Kw in about 2 hours of driving. Volta seems to recommend using the Coleman Mach 48v AC unit with their system. It's kind of big and bulky on the roof. It's works, but it's not the quietest or most efficient thing out there. I can run it for at least 10 hours off the battery pack. In general I really like the Volta system, but I think you can build something comparable on your own for a lot less money. They have, however, done all the engineering for you.