Thanks to ChrisLi for the reply, which was spot on.

Just to follow up for any who might face something similar in the future. The bus to the international terminal from domestic is similar to the one that runs between T1 and T2 at Narita. If you have luggage, they don't have a rack in the bus, so I just stacked my bags on the floor. Fortunately not too many riders so I didn't get in anyone's way. Some buses only go between the two domestic terminals, while others also go to the international terminal. They stop at the same numbered stop, so it might be something to look for.

The international terminal is indeed not so large. I stored my bags at a counter set up for that purpose on the 3F close to the check in counters. Cost is 500 yen per bag per 24 hours.

From the international terminal, it was easy to find the buses to various parts of greater Tokyo/Yokohama, etc.