Tow ratings are related to overall trailer weight, not trailer size, in most cases. However, some tow ratings impose limits as to frontal area of the trailer and overall length. Tow ratings are determined by the vehicle manufacturer according to several criteria, including engine size, transmission, axle ratio, brakes, chassis, cooling systems and other special equipment. The overwhelming majority of RVs utilize electric trailer brakes that are actuated when the tow vehicle's brakes are operated, or when a brake controller is manually activated. A heat exchanger similar to a small radiator through which automatic transmission fluid passes and is cooled by airflow.
Travel trailers are available with one, two or three axles.
Depending upon tow ratings, conventional trailers can be towed by trucks, cars or sport-utility vehicles. The wiring harness that connects the tow vehicle to the trailer, supplying electricity to the trailer's clearance and brake lights, electric brakes and a 12-volt DC power line to charge the trailer's batteries. An umbilical cord can also be the power cable that is used to connect to campground 120-volt AC electrical hookups. Following government prescribed test procedures, tire manufacturers perform their own evaluations for these characteristics. Each manufacturer then labels the tire, according to grade.