Don't forget to add the addon so that the trailer has something to connect to.
An example for the 4310 with a trailer hauling garage points would be:
Type kamaz4310 <---- This is the truck.
Addons kraz_tractor <----- This is the addon to allow the trailer to connect.
Trailer trailer_tent <----- This is the trailer.
Coffee hasn't kicked in yet.
In that case, I'm not sure. I could never seem to get just a trailer to spawn. It always had to be connected to a truck.
My workaround for this issue was to also add a truck but to spawn the truck either out of fuel, or damaged out, or both so that it couldn't be used. At least, not easily.