Here is a model I created some time ago but never shared. It is based, as you see of most normal trailers that you see carrying heavy equipment or other various items. This took about 25 hours to make in LDraw. The goose neck detach just as in real life to allow equipment to roll on to the trailer. The trailer has full "Steel" frame from beginning to end to support items of various weight. It has a recessed area in the middle for and large equipment that needs to fold like excavators. The 3 axles are all spring loaded, with the last last axle having a small pneumatic cylinder to pull it out of service. The pump and valve are located just in front of the axles. This model should respectivly hold and fit model #8275 (Dozer with Power Functions) without issue.
The model is 129 studs (40 inches) long, 17 studs wide (5.23 inches) wide, 12 studs (3.69 inches) high.
Here you can see the valve for lowering and raising the third axle out or into service. My attempt to mimic the frame and supports for the is seen as well.
The recessed area can be seen clearer here. Also you can see the full "Steel" frame as well.
Here is the pump for raising the axle out or into service. The trailer is setup for all lights for awareness of heavy loads.