Just curious...how fast was traffic going? It's generally not a good idea to get very close to dump trucks. They'll squoosh a car.
I will say this. If this was a crawl type merge, you might have a prayer...if you had started the move over and he rolled up into you as you were stopped.
ALSO, if he hit you from behind, even one corner, then he's usually presumed to be at fault.
HOWEVER, there's no one size fits all answer. My uncle was hit as he pulled out from a side road, which would normally make him at fault, but the car that hit him had come flying over the crest of a hill, well in excess of the speed limit. If you can prove that someone else was not obeying the law, it sure makes life easier for you.
Are you on good terms with your car insurance guy? He might be able to give some advice...even if you don't plan to file a claim. Hey, you give them a lot of money. Get something back for it.