You want my honest oppinion so I will give it to ya
Personally I feel the man driving the truck is responsible for the crew onboard and equipment being hauled.
I say if it was a complete accident,unavoidable,and had no insurance on trailer then all will be responsible.But the driver who had ample time to stop,slow down or whatever his actions permitted is responsible for the trailer.
Why was the trailer being used without insurance in the first place?
6 men should have know to check it beforehand.What would happen if the Trailer came loose and caused serious injury to another motorist?
I would think that the mans truck insurance will cover the expenses for the trailer.
Just be forwarned he wont come after the 5 of you guys for a little compensation for his insurance.
Another thought-----was this guy aware of the rock on the road?
Was he warned buy any of the passengers?
Maybe he was tired and never clued in till the last minute.
Im sure the truck insurance will cover it though.