So... if you put up said metal building, stash the truck inside it and close the door... I'm just not sure how that does not solve the problem. Unless they came onto your property and opened the door, they would have to assume that it's gone, right? As far as they're concerned it's not even there anymore. It's none of their business what you keep inside your property and out of sight.
Rapid City has an ordinance for vehicles with no plates, expired plates, obviously inoperable, etc...sometimes they get after people for parking on the grass in their yard too. Nit picky stuff that is selectively, and rarely, enforced. I live just outside city limits and will probably get annexed eventually, and I have several vehicles that don't have plates on them. If I get annexed into their little commune, you can bet the No Trespassing signs are going up... what are they gonna do? Send a tow truck driver over to open my gate and remove things from my property? That'll be fun for them.
I'm fine with keeping my place looking decent, I appreciate when my neighbors do that for me as well, there is a point where people need to be left alone though.