I'm no lawyer, by any means, nor do I know anything in particular....

but don't they have the burden of proof? Most times when there is a part that failed and they are refusing to honor a warranty, they provide specific reasons.

If the tank was hit, there would be an impact mark. If the truck was abused there would be other proof: alignment, bent rim, broken exhaust ... something.

I think you can demand a letter from them explaining in detail the exact reasons your warranty is being denied "so that your lawyer can review it and file a grievance" they may be more apt to honor the warranty than to go thru the work of explaining why it is being denied and then get sued over it … they are taking the risk of being wrong, you on the other hand have nothing to lose.

Usually if strong language is used in a calm authoritative manner, positive results ensue, particularly when one demands exacting reasons to be in writing.

I get told no quite often, comes with the job, but I rarely let the issue die there.

just a thought.