It sounds like you have a really heavy workload, and are under-appreciated. However, on the plus side, from what you say you are vital to the business, and that can be good for you.
Having been a manager of a small team for a few years now, I can say that sometimes those in managerial positions will not fully understand the workload you cover and its importance, your flexibility at key times etc. I realise this might be giving your management too much credit!
However, I do always appreciate people voicing their concerns (certainly rather than leaving!), but this has to be in a measured, evidenced way and at a good time i.e. ask to book in a meeting in a quiet place etc, giving some time (a week?) to prepare, and let them know loosely that you'd like to discuss workload and support.
If you can go in with a calm approach, saying how challenging the job is (and maybe you enjoy that and aren't bored), what your responsibilites have become, how this will continue etc, and what hours you are working. An effective conversation will begin with you airing your issues, and how they affect your work / home life, saying how you want to ensure the company is doing OK, but might need some support. If you have potential solutions (interns, apprentices etc), even better.
I hope you don't mind the long reply, and I'm sure you might have tried some of these things...
Best of luck, Mat