Call the police, open a report of vandalization. Call your insurance company. Have the police collect soil samples at your residence, anywhere you have been in the recent months, and at the dealer that denied the claim. Make sure they question the mechanics that worked on it. If they cant find out who put the dirt in your still entitled to the insurance claim. If they find out you get the truck fixed and someone goes to jail. It's a win/ win-win situation.
If you or someone you know put the dirt in and you do this it is insurance fraud. I'm not accusing you, just don't want to give advice that could hurt someone.
You pay insurance for collision and theft, it does cover damage from vandals as well. Use it.