Here's my advice.
Don't work a day job.
Or if you have to, set a time period like 6 or 9 months. In that time, pay off as much debt as you can, and figure out how to leave as lean and tight as you can.
At some point, if you are going to do this full time, as your career - it needs to pay you, as a career does. If you give yourself an out - like a day job - it allows you to take either take low/no-paying jobs. These can be great for building up your resume, but eventually you need to make money at this.
You also run the risk of discovering that indeed, you day job is infinitely more appealing then stage managing - and the day job will take over your life - why? because it's easier and more secure - and chances are, more lucrative.
If you can't get enough money to pay for your life by stage management, then, to be brutally honest, perhaps you should choose another career - and just stage manage as a hobby. The job market has a way to tell if you are good enough to make in the biz.
Now, if you want to work in the field with a day job . . . then things like box office, office admin for a theater, rentals, etc would work. Or, just find a nice office temp job.
My last day job was for a health care management company (we administrated a nation wide HMO locally). I was a temp, but survived three rounds of lay-offs, and was eventually offered a full-time management position. I went out to lunch with everyone, and asked the rest of the staff I worked with, what they wanted to be when they grew up - a writer, a musician, an architect - but all had been side tracked by their day job which turned into the real career. I made the decision then and there that stage management was going to be my career and have never worked a day job since - that was 16 years ago. Now, once you yank away the security net of a day job, you hustle your a** off to get a job. You start planning long term.
It hasn't been easy. But, it was the only way - I was tempted once to quit this career - it's a hard damn career to make a living at.