Resume advice: 1 job at same company for 28 years

Highlight multi-year periods where you have one main role, or where you were the only one handling some recognizably major slice of the business.
Each phase gets its own resume entry.  Ideally, these will show progressively more responsibility and capability, as you grew at that company.  Put in a recognizable title for each entry -- you're trying to get across quickly what each phase covered.
One colleague showed me his, with a similar path to yours.  He had a small introductory section explaining the size and why that size didn't change much across the years (owner's choice), and that as one of only a dozen, many of his roles continued throughout.  Yet he still chose to separate out periods with distinct focuses, such as what he verbally termed his 'year of marketing' (lead marketer) and his 'three-year slog through sales' (business developer) and 'those three quarters standing in for the owner as she recovered from a major illness' (acting CEO).  Each had their own bullet points, with occasional accomplishments which didn't fit the period's theme, yet demonstrated ongoing notable successes.

Bradley- were hosting a free webinar about resumes this month if that would be useful to you! You'll be able to the see the sturcture and intent that would help to format even these less specific timlines and roles. Register here: https://www.manager-tools.com/your-resume-ready-webinar