Can I Tow a 5th Wheel Travel Trailer with a Semi Truck or 18-Wheeler  

Question:

Expert Reply:

The king pin dimensions can be the same between fifth wheel trailers and large commercial trailers. Both types of trailers often use a standard 2 inch king pin although semis sometimes have a 3-1/2 inch. As long as your truck fits a 2 inch king pin it can work. Semi trucks were not technically designed to tow camping style fifth wheel trailers, so there are several factors that you will want to take into consideration before attempting to tow.

First, you will want to make sure the truck can tow the trailer level. Towing the trailer with the nose too high or too low can cause issues with sway, stability and tracking, so it is best to tow the trailer as level as possible. To determine if there will be a height issue, you can measure from the ground to the top of the hitch on your semi truck and from the ground to the bottom of the pin box on the fifth wheel trailer. From there you can compare the measurements. I recommend having the truck and trailer on level ground before measuring.

Another factor to consider is the wiring on the truck and trailer. Semi trucks typically have 7-pin round socket, similar to part # PK11720. Fifth wheel trailers only have one RV style female 7-way blade style trailer connectors like part # PK12706 (see photo). Semi trucks and commercial trailers also operate on a separate wiring system, meaning the brake light signal and the turn signals are on different circuits. Fifth wheel trailers use 2-wire systems with a combined brake light and turn signal circuit.

Also, most fifth wheel trailers have electric drum brakes and they are wired to the single RV style 7-way connector on the trailer. Commercial trucks and trailer will typically have a connector for the trailer lights, but a separate connection or air brake or hydraulic brakes on the trailer. To actuate electric trailer brakes, you will need an electric brake controller like part # 90195 in the cab of the truck wired to a dedicated 7-way socket for the fifth wheel trailer.

I am not aware of any adapter on the recreational towing market that will convert a semi truck connector to an RV style connector, but we do carry the necessary parts to equip your tractor to tow a fifth wheel trailer using the following parts:

-Tail light converter # 119250KIT - to combine the tail light circuits on the semi truck.
-Universal 4-way to 7-way Installation Kit # ETBC7 - to give your truck a 7-way RV style trailer connector. This kit also includes all hardware necessary to install a brake controller.
-Prodigy P3 Brake Controller # 90195 - to actuate electric trailer brakes.
*You may also need additional amounts of 16 gauge (# 16-4B-1) and 12 gauge (# 12-1-1) wire to complete the installation.

I have attached a trailer wiring FAQ for you to check out as well as a diagram showing how a RV style 7-way connector is wired.

Finally, if the leveling and wiring concerns are worked out, you will want to make sure you will have enough turning clearance so the truck and the trailer do not collide. There are two clearances that you will want to be concerned with, truck cab to trailer overhang clearance and truck frame/wheels to trailer clearance.

To make sure you have enough cab to trailer overhang clearance, you will want to measure from the center of the fifth wheel hitch to the back of the truck cab. This distance must be 4 inches greater than half the width of your trailer. For example, if your trailer is 96 inches wide, the distance from the fifth wheel hitch to the truck cab must be 52 inches (half of 96 is 48 and 48+4=52).

To find the truck frame/wheels to trailer clearance, you will want to measure from the center of your fifth wheel hitch to the back corner of your frame/wheels (see photo). Next, measure from the center of the king pin on your trailer to the underside of the trailer. The measurement on the trailer must be 5 inches greater than the measurement on your truck for proper turning clearance.

expert reply by: John H

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