Taking questions one at a time
Will the generator / Intella power 9100 charge the batteries: Yes
How long: Depends
Should I buy a 3 stage? That is what it depends on.. Question: On the 9100 series there is an optional 'Dongle" that plugs in kind of like a modular telephone (There is a screw involved as well) this is a small square box with a button and a LED and a picture of a wizard on it. Do you have such a dongle plugged into your 9100... IF so you HAVE a 3 stage plus converter, one of the best on the market.. If not you can get the dongle for less than 50 bucks (Way less) at most RV stores. Instalation is simle, remove one screw, remove dummy plug, insert dongle's plug, replace screw.
Now back to how long: This depends on the ratio of converter to batteries. Charging happens in TWO of the 3 stages.
BULK, Bulk charge is a high speed charge for batteries that are hungry.. Let's assume you have 200 amp hours of battery, they are at 50% state of charge, and your converter is a 50 amp
You need `100 amp hours, with charging loss this means 2-2.5 hours to get the batteries up to 90 percent
NOTE: larger or smaller, batteries or converter, Do the simple division, assume you need to bring them up to 100% in BLUK and you will be right close.. You won't be fully charged at the end of that period though
Then we switch to absorption .. this is the last 10% it will take about 2-3 hours. No way around it. So in t his example 4-5 ours for a full recharge.
At this point the Wizard (The dongle) should switch to float, after 20 hours of floating, it should go to Equalize" for like 15-20 minutes. (That is the plus I mentioned earlier)
This is, as I said, one of the best converters made.. I am very happy with mine (Intella Power 9280 with wizard).