Proper way of referencing ranges of chapters
According to Wiktionary and a number of other sites, "chs." seems to be the appropriate pluralization of the "ch." abbreviation for chapters.  Note, however, that either is likely to be quite intelligible to a reader, and if the typesetters disagree with your choice, they'll surely fix it to their preferred style.  In short: just use "chs." and don't worry about it.