I think "engineer" is the best description for what you do though - and I'd argue that your experience far outweighs the paper that some might think gives you the right to use the title. It would instantly eliminate the confusion about you being a salesperson that will come with just about any other title you choose. Don't let your modesty/humility prevent you from giving yourself a title that best describes your services.That has always been a problem, I can't come up with a short and yet descriptive name, you can thank Joe Aquilante for the "tire specialist" moniker.
No matter what I come up with, someone always thinks I sell tires, and I refuse to be called an "engineer" I don't have the degree and I'm not a poser.