My guess is that this is where you simply need a lawyer - at least to write a letter that spells out the rights you're asserting and makes all the correct threats, without any accidental mis-statements that might muck things up later. I suspect they're just making a ploy - they're hoping you'll say or do the wrong thing or decide it's not worth the bother.

You might also want to talk to your state's attorney general office and inquire about what one does when hit by a truck not being operated within legal guidelines. The state or federal DOT might get involved - that might cause an investigation that might remove the need for you to prove anything about where the truck was.