Here is the full step by step instruction of my truck with trailer. More pictures are on https://www.instructables.com/id/Knex-Truck-with-Trailer/
At first the trailer step 1 till 3. And then the truck step 3 till 6.
Step 1: Trailer
At first the trailer axle for the wheels. Build it and put together. Per axle 2 blue spacers and 2 large wheels.
Step 2: Trailer
Next: Build the trailer back, trailer front and the trailer legs.
Step 3: Trailer
Build the trailer. Let a space free for the legs. Add the back, front and legs from step 2 to the trailer. Add the axle of wheels from step 1 to the white rods.
Step 4: Truck
And now the truck step by step. At first the steering.
Step 5: Truck
Now build the rear axle with the mechanism to attach the trailer at the truck. And add all to the steering from step 4.
Step 6: Truck
And the last step. Always alternately: build and add to the model.