Drivetime Drivetime is screwing me Internet

I bought a Ford F-150 from Drivetime. I had the truck for a year and a half and then it caught fire while driving it. It burned so bad it was considered a total loss. I did not have gap so I was left owing over $6000. When I called to report the fire I was told that my truck was still under warranty.
The warranty was 36k or three years. I had put 15k miles on the truck. Now Drivetime is telling me that because there is no collateral that they had to puta $6200 charge off on my credit. I had never been late on a payment.
The truck was burned so badly that it was not possible to determine what started the fire. Since they couldn't determine what caused it to burn Drivetime will not honor the warranty. I don't know what to do. Drivetime said they will sell me another vehicle but after this experience I don't want to deal with them anymore. Any advice will be appreciated.