I sent an email to the town with the state statute and trailer definition and got the following response:
The DMV has very limited descriptions for “trailers” We have campers that are registered as utility trailers. There are only two codes under DMV for any type of trailer, code 11 and code 31. So you see how it would be very difficult to extact. The use can not be commercial at all and the gross weight was an average that was talked about a lot when this law was first passed. The law is silent on how to describe a utility trailer so it has been put in the hands of each assessor to determine those parameters and be consistent to their taxpayers. A lot of towns are using gross weight or asking for photos of the trailers to determine the actual type of trailer. You are able to go to the Board of Assessment Appeals in September to appeal this issue. As of right now, I have to be consistent and stick to the parameters that I used when issuing an assessment on any and all trailers registered to Derby. If you would like me to send you information for an appeal, let me know.
So, looks like I'll still be paying taxes on it.