Those of us who teach Victorian fiction face this problem, too. After all, you’re not really teaching the Victorian novel if you don’t have at least one triple-decker on the syllabus.
Typically, I start very small, encouraging students to treat passages almost as if they occurred in lyric poems. We also talk about narrative conventions, and the way writers flirt with, exploit, or thwart them. This gives them something concrete to discuss (even if it’s the slipperiness of a metaphor, etc.).
Eventually, enough instances accrete, and there’s a sort of tipping point five- or six-hundred pages in where we can start putting things together. (And still have another 400 pages to go!)
It is perhaps a bit easier in the Victorian novel, which helps set up conventions that Pynchon, DeLillo, et al., play with, but I would probably work in that direction.