I would say the weight of the trailer is used. Look at real life, if I side swipe a little, light weight, trailer I’m likely to do more damage than if I hit a heavy one.
Plus Rule of Carnage applies – more damage is the way forward 🙂
I have always used the weight of the “cab” as that is how war rigs worked. I don’t recall it ever coming up when we were testing Rusty.
Rule of Carnage doesn’t really apply here, but if you wanted to try and use it I would go the opposite way to Lunchmoney if I smash into a heavy thing more damage happens (although more of it would be to me, but rule of carnage doesn’t distinguish ). The reason I don’t think you can use it is because there are cases when counting the trailer as light does less damage, and cases when counting the trailer as heavy does less damage depending on the exact details of the crash, and the rule ought to be consistent