Say that I am towing my friend's new travel trailer with my pickup and hit a low hanging tree limb and destroy the trailer's electric awning. Whose insurance pays, minus the deductible, of course. ...
Your truck policy wouldn't cover it even if was your trailer. If the trailer hit a parked car then it would cover damage to the car, but not damage to the trailer. That is what your trailer collision policy is for. I would suspect your friend's trailer collision policy would cover the accident even if you were towing it. Let us know how it all turns out. No good deed goes unpunished.