Most of us haul personal items with personal pickup trucks. It is possible it is different if you are using it for a business.
Here in Oregon, there are two types of trailer licenses.
Under 8,000 lbs, regular trailer plate.
Over 8,000 lbs, permanent plate. With that, the trailer is part of the truck weight licensing, and it requires "T" plates on the truck.
I assume your trailer had working trailer lights, and electric brakes.
My guess is you have less than $1000 in fines and it will be easier to pay it then to fight it out of town. However, if it was a personal thing for myself, I'd be tempted to fight the charges. A lot sounds like complete BS from a junior officer that is still learning the ropes.
A good fire extinguisher is a smart idea.