Ron,
I can't tell much about that alternator in your link. The brand is stated as "DB Electrical", so it's most likely not originally a Ford OEM alternator that's been rebuilt to a higher specification.
It it was me, I'd go the Ford OEM route. I wouldn't be too concerned about any wiring issues between the OEM alternator and your batteries. However, you might want to contact a PC technical person to see what they say about how they connected into the E350's electrical system with the coach battery system.
What makes you think that you need to change your alternator? Your 115 amp version will probably work just fine. Your battery bank from the two 6V AGM batteries in series will only be 220 amp hours. My AGM 12V battery bank is 200 amp hours and our Ford 130 amp alternator has been just fine with them for years. Our alternator has spiked up to around 50 amps into the batteries - with the engine idling - when our batteries were low from camping, but this only lasted a very short time. When our 200 amp AGM bank is low what charges them fast is the steady 25-35 amps the alternator supplies to them in 3-5 hours of driving down the road between campsites - this is what can bring them up to full charge nicely for the next campsite.
What you might want to do is install a voltmeter (four decimal place) and ammeter (three decimal place) for your coach batteries right on your dash (I sticky-back Velcro'd them to the dash surface) so that you know what's going on with them all the time. These meters will show continuously what voltage is on their terminals, and how much current is going into them (positive ammeter readings) or out of them (negative ammeter readings) them. This monitoring has been very invaluable to me.
You may be just fine with your existing alternator ... check with PC for the final word. You have probably brought in a lot of new customers for them over the years - they owe you.
