Yes, you'd file a DBA, probably with some kind of town/city clerk, although it's probably state specific. If you end up with an LLC then you wouldn't need a DBA unless you want to refer to the business as something other than the name of the LLC.
And I wasn't necessarily saying an LLC is a bad idea. For example, if you install a machine, everything is in good working order, you inspect it regularly, and follow whatever industry standards and some kid decides to climb the machine and it fall on them, maybe having an LLC would be a good idea. Probably worth talking with a lawyer to see what they recommend.
What I'm talking about is more like you're driving to check on a machine and you get in an accident (this would also apply if you did something negligent like put expired food in the machine). They're definitely going to sue you personally. On the other hand, if you had an employee and they got in an accident through no fault of yours then that's where a business structure really helps.