Several of us on the same train of thought. The list of suspects should be every employee you have or that has left for any reason since you set up this shop with the trailer in it. It has to be someone VERY familiar with your set up and operation.
These are the times that try the relationship between employer and employee, because you need to get very quiet and removed while investigators treat everyone, regardless of how you feel about them, as if they are the one.
When, if, they find the crook, you may be shocked at who the backstabber is.