DoubleA,
I was one of the two guys who worked to get that law passed. DOT actually wrote it, but we seized the wording and got it enacted. Porkysplace is correct that it may end up being a judge who decides what the law means. The pertinent statute is [s.341.269, Wis. Stats.], and this is the exact wording:
A vehicle registered under this section may only be used for special occasions such as display and parade purposes, club activities, exhibitions, and tours, including traveling to and from such events, for necessary testing, maintenance, and storage purposes, and for occasional personal use, but not for regular transportation.
This means that you cannot use a HMV-licensed truck for commercial purposes, nor drive it on a daily basis. It does not specify a number of times per year that you can use it. The cop is wrong.
Importantly, in Wisconsin, you have the right to appeal things like this through the Department of Administration, division of hearings and appeals. Here is a link to the Wis. DOA:
http://www.doa.state.wi.us/Divisions/Hearings-and-Appeals This is how we beat WisDOT over issues like whether a 1985 Chevy Blazer can be registered (it can, and we fought a 19 month DOA court battle to prove it). You don't even have to hire a lawyer, although you may.
So, go and use that truck for "occasional personal use", carry a copy of that Wis. statute printed out and give it to him, and if you are threatened with a ticket, be polite and explain to the cop that you will be filing an appeal. Then call me and we will help you fight it. As the guys who essentially wrote the law, we can testify as to the intent of the statute.
Unless this truck is your only form of transportation (do you have a "daily driver"?), it should not be considered your regular transportation. Your "daily driver" is your regular transportation.
I'm sending you a pm with my home info.
Regards,
Paul Underwood
edit to highlight statute in red