Hi Lawrence,
I do think RickRoll's advice was good, and the first thing I would say is that you should leave those 301 redirects in place, on a page-to-page level, for a good long while - don't change them again. It looks like there are still some pages from boston.renthop.com in the index, which indicates that Google may not fully understand that these pages have all moved. There are only a few still in there, though, so I don't think that's the only issue.
One thing that can happen when pages 404 for a while is that even if you 301 them later, the link equity of the original pages can disappear (it doesn't always, but it can). So you probably want to spend some time building links to these pages/site sections, and where you can, I also recommend reaching out to sites that linked to the old subdomains and asking them to update the links. I'd recommend trying to get new links from local-specific sites (i.e. sites about Boston or Chicago, vs. nationwide/non-local sites about apartment hunting), and here's why:
In my experience, Google usually treats "{keyword} in {location name}" searches differently depending on whether or not you're actually in the location you're searching for. If you're not in the location, it treats {location name} as a keyword, but if you are in the location, it treats {location name} as a location. So for example, if you search "apartments in Boston" and you're not in Boston, it will show you pages that rank for the term "apartments in Boston" - but if you are in the Boston area, it will show you pages that rank for the term "apartments" and that have strong local indicators that they are in Boston, or more specifically, are near to you.
So since you rank nationally but not locally for this term, it sounds like the signals you need to focus on improving are local signals. You can do this by building links from local sites, by adding e.g. "Boston, MA" to title tags on individual apartment listings, and by building out some unique, locally-focused content like neighborhood guides to add value to searchers from that area.
I hope that helps!