The rental contracts used to forbid travel on "unmaintained roads" but now they say "unpaved". Your failure to read the agreement is no defense I'm afraid. You haven't told us enough to suggest other possibly creative solutions. I will say that when I drive a rental on unpaved roads, I do a fair amount of work to clean out the dirt so it doesn't show on return. On a dusty road, you get dust around all the door seals, sometimes inside the compartment around the gas cap, and in the wheel rims. That being said, if I damage the car on an unpaved road, it's clearly my responsibility if my insurance questions violation of rental terms. The cleanup is mostly so the rental companies won't get mad at me, or charge me extra for a deep clean.