Scratched rental Hummer...?

I rented a 2008 Hummer for work and the rental car company gave me it for a full size car price bc I am frequently renting from them. Here is the problem, I drove it across a relatively open field an it got muddy and scratched. I DID TAKE THE INSURANCE as I always do. They however said it isn't covered bc the vehicle was taken offroad... They said I am negligent. Now the quote is saying it will cost 1500 to repair. I am not sure why the insurance would not cover this. Right now they are debating internally if it will be covered. What can I do and should I fight it with my insurance company? I thought that was the point of having trucks, 4x4s to rent etc. to use for hauling things and most of the time that is offroad....