While I see that this thread focuses on campers in Washington State, I live in Oregon and owned 2 FWC's over a 20 year period. The first was a FWC Grandby with bed, sink, cooktop and heater. I registered it with the state and mounted the plate on the rear. The 2nd one was a Grandby Shell, which I had for 9 years. I never registered it with the state. I decided that after reading the the Oregon DMV definition of a camper. On the Oregon DMV website it specifies that a camper "has a floor, is mounted on a motor vehicle, but not permanently attached, and is designed to provide facilities of for human habitation defined as "permanent sleeping and cooking facilities". By that definition, there was no arguing that my first fully outfitted FWC was indeed a "camper". However, my shell only had a bed and a few built in cabinets. Given that, I never registered it and never encountered an issue.
I did do some basic modifications to the shell by installing a shelf between cabinets on the drivers side to hold a 2 burner cooktop camp stove that was "temporarily" fixed in place by straps when underway. I had a propane hose running outside to connect to a propane tank when needed. Did not have running water, as I used 5 gallon jugs with a spigot and a tub as a sink. Also had a Lagun table for internal use and compact folding "bistro chairs" for seating. It worked well for my wife and I for 9 years and provided much more open space inside the camper for an 80lb dog and two people moving around as well as hauling stuff inside when needed. Not a solution for everyone though.
It may be useful to see how Washington defines a camper. While it sounds like the owner of this thread has a full feature Hawk, there's likely no getting around registering his unit. However, for anyone mounting an ATC or FWC shell on their truck that is not fully built out, there may be some wiggle room. Having said that, I was never stopped and challenged by a LEO claiming my shell had to be registered. If I had been, I may have had to present a persuasive argument that it wasn't a "camper" by Oregon DMV standards. I kept a printout of the DMV regulations in the trucks cab just in case.