We used a oil filled space heater. They are very safe and inexpensive and do well heating a small space. He might insulate his windows and block off the cab with either bubble wrap or reflectix to help hold the heat in. We were caught temporarily last November by low temps and had to heat ours for a couple of weeks until we could winterize. We used the 30 amp line to a garage outlet and a converter to a 20 amp circuit. Ran more than one heater. One in the bathroom and one in the main room. We were not also living in it though.