I'm Neal and I am here to assist you. Only accept when you are happy with my assistance. If I can't help I will open your Question to other experts.
Is this the adjuster for your insurance company or the other drivers company?
You are really going to need to get the dealer or a body shop that will confirn this damage could have been caused by the accident and even something from the dealer confirming the condition of the truck when it was sold.
If you have something to actually support you assertion you should be able to get the insurance company to reconsider their position. You should also talk to your local agent about the situation and see if they can help as they are supposed to be your advocate.