Chassis and engines in plain words

A motorhome is two things joined together: a house to live in and a vehicle that carries it. This guide explains the vehicle part in plain words.

What a chassis is

A chassis is the load-bearing frame of a vehicle. A rolling chassis is that frame plus the running gear, such as the engine, the transmission (the gearbox) and the suspension (the parts that cushion the ride).

Vehicle makers sell chassis that other companies finish with their own bodies, and motorhomes are one of those body types.

Who builds chassis for luxury coaches

• Spartan RV Chassis, of Charlotte, Michigan. Spartan started in 1975 with fire truck chassis and had entered the RV market by 1985. Spartan RV Chassis is part of the Aebi Schmidt Group today. Its website lists chassis from the K1 360 to the K4 605 and names Newmar, Entegra, Foretravel and Jayco as coach builders. The 2026 Newmar King Aire rides on the K3 605.
• Freightliner Custom Chassis Corporation (FCCC), of Gaffney, South Carolina. FCCC is part of Daimler Truck North America. It describes itself as a maker of diesel chassis for motorhomes.
• Tiffin's PowerGlide. Tiffin calls it its exclusive chassis, standard under the 2027 Phaeton, Allegro Bus and Zephyr. An FCCC news release from January 2025 says the new PowerGlide was developed in partnership with FCCC. The sources we read do not say who builds it.
• Newell Coach. Newell says it has designed its own chassis and body since 1967, and that it is one of the last coach makers to build both from the ground up.
• Foretravel. The 2026 Realm FS605 rides on Spartan's K4 605 chassis. The 2027 Realm Limited FL605 joins an FCCC chassis to Foretravel's own structure, called the ForeTruss FL chassis bridge.
• Prevost. For a bus conversion, the bus maker supplies a whole bus, called a shell, and another company finishes the inside. Prevost offers its H3-45 and X3-45 as conversion shells.
• Truck makers. A Super C is a motorhome on a truck chassis that keeps its cab. The 2027 Renegade XL is offered on a Freightliner Cascadia or a Peterbilt 567. The 2027 Newmar Supreme Aire uses a Freightliner Cascadia. Dynamax lists Freightliner Cascadia and M2 chassis among those under its motorhomes.

The engines

Engine size is given in liters, which measure the space inside the cylinders. The builders above name these engines:

• Cummins L9: an 8.9-liter diesel with six cylinders in a row. Tiffin lists it at 450 horsepower (a measure of engine power) in the 2027 Phaeton and Allegro Bus. Around 2016 to 2017, Cummins shortened the name from ISL9 to L9, so used coaches may carry the older name.
• Cummins X15: a 14.9-liter six-cylinder diesel. The King Aire, the Newell and the Foretravel Realm Limited FL605 list it at 605 horsepower and 1,950 pound-feet of torque (twisting force). It replaced the older ISX15 in 2016 and 2017.
• Volvo D13: Prevost has used this engine from Volvo, its parent company, since the 2008 model year.
• Detroit DD13 and DD16: the 2027 Renegade XL offers these with its Freightliner chassis, at 525 and 600 horsepower.

Used coaches can have engines that the new coaches above do not list:

• Detroit Diesel Series 60: Prevost used this engine until the Volvo D13 replaced it.
• Caterpillar: the 7.2-liter Caterpillar 3126 arrived in 1995 and became the C7 in 2003. It was used in RVs as well as trucks and buses. In June 2008, Caterpillar said it would leave the U.S. market for on-highway diesel engines before new 2010 emission rules, which limit exhaust pollution, took effect.

The transmission

The transmission is the gearbox between the engine and the wheels. Allison Transmission, of Indianapolis, makes automatic transmissions for commercial vehicles. Its motorhome versions carry the letters MH.

The King Aire, the Newell and the Foretravel FL605 all pair the 605-horsepower X15 with an Allison 4000 series transmission.

Words a buyer will hear

• Diesel pusher: a motorhome with its engine in the back is called a pusher. Pushers generally burn diesel fuel.
• Drive axle: an axle holds the wheels on each side of the vehicle. The drive axle is the one the engine turns.
• Tag axle: an extra axle behind the drive axle on a long, heavy coach. The engine does not turn it, and it helps carry weight. Tiffin rates its tag axles at 10,000 to 14,000 pounds. On the 2027 Newmar Supreme Aire, only floor plans 43 feet and longer have one. Some tag axles can steer. Spartan fits a passive steering one, and Newell fits an active rear-steer one.
• Torque: the twisting force an engine makes, measured in pound-feet.
• Horsepower: engine power. It equals torque times engine speed in revolutions per minute (rpm), divided by 5,252.

What the numbers mean for a buyer

Engine numbers alone do not tell you what a coach can pull. The 2026 King Aire lists 605 horsepower and 1,950 pound-feet, and its chassis is rated to tow 20,000 pounds. The 2027 Renegade XL lists a little less torque, 1,850 pound-feet, but it is rated to tow 30,000 pounds. So check the tow rating itself, not just the engine.

Look at engine speed too. The X15 in the King Aire makes its full torque at 1,000 rpm and its full horsepower at 1,800 rpm.