I think you're doing all the right things, and you're learning exactly what you set out to do. Unfortunately, the data is not telling you what you wanted to hear.

Keep in mind that running customer development experiments, it's ok to hope that your thesis is validated, the point of the experiment is to learn and iterate. If you just go looking for "proof" you'll find a "smoking gun" out of anything, but if you stay dispassionate, you'll find something worth pursuing.

Granted, 4 is a statistically insignificant sample-set but what you're learning is that food truck operators are busy operating their business and are going to be hard signing-up for any kind of service or app.

Your messaging might be wrong too, but based on some experience in this space, I think what you're finding out is your idea might just not be able to find traction here.

Look to your other market, experiment, be open-minded to the results and keep trying.

I'm happy to talk to you in a brief call to help you setup your experiments with a bit more clarity and talk about how you can do outreach to get a more meaningful sample/response rate.