Don't be constrained to a conventional "chronological resume" where you list: Job A, dates, responsibilities. Job B, dates, responsibilities. etc.
You might consider instead a "functional resume", where the highlight is your skills and responsibilities. Your actual employers in secondary. (A quick google search or a book at your library should provide plenty of examples). Emphasize verbs/buzzwords like Manage, Coordinate, Supervise, Facilitate, Communicate, Organize (which any job will value). As a stage manager you are skilled as managing and organizing people as well as data. You are responsible for communicating information between artistic and technical departments. You are responsible for quickly assessing and resolving situations as they arise. (Think in general terms and build from there.)
As for your employment history, you might list it by profession/position, ie.:
Stage Manager, 1996 to present.
* Such and Such Theatre
* So and So Theatre
* City Theatre
Lightboard Operator, 1994-1997
* Repertory Theatre of My City
* City Theatre
Box Office Agent, 1994-1996
* Such and Such Theatre
Another option... Start with some temp agency work. They'll be less discriminating, can even offer some resume help and will give you some non-theatre work experience quickly for your resume.
Give employers a little credit, especially if you are applying to smaller businesses (ie. a place where an actual person will look at your resume). They may not work in theatre themselves, but a lot of people GO to theatre and have a basic understanding. Don't assume their preconceptions about our job will be negative.