They don't like the fact you have extra vehicles on the property? Or do they object that they're not tagged? If it's a simple issue of having "registered" vehicles on the property see if you can register them inexpensively as classic, antique, special interest or whatever your state has available for collector cars. If insurance is mandatory you can probably get cheap insurance and register them for a month just for the sake of having a plate on them and making them legal. Depending on your state it might be cheaper to simply store them somewhere, but a guy I talked to once had to register a parts car as an antique just to get the city off his back.
Portable garages may satisfy the insurance company, or they may not since they are "temporary structures" High winds and tree branches ruin them easily. Maybe ask around if someone has a car trailer or tow dolly to borrow to save the tow fee. As a last ditch, talk to the tow guys directly. Some of them are car guys and LOVE old cars. They may give you a cash deal break just so they can say they had a Challenger on their truck.