Service truck got stuck in our muddy field. Who covers the cost of the tow truck?

You are responsible for the towing cost. The rental company's driver attempted access after your warning, but the service contract places the duty of providing safe, passable access on you, the customer. Most contracts state that the customer must ensure truck access to the units on every service day, including maintaining roads and level ground that can support a heavy vacuum truck. A blocked or inaccessible unit typically results in a missed service charge, but if the driver attempts access and the truck gets stuck, the customer is liable for recovery costs. In my experience, even with a warning, the driver may still be dispatched per company policy, and the contract language holds. Review your rental agreement for the access and liability clause; it will specify your responsibility for maintaining access routes. For future services after heavy rain, consider canceling or rescheduling if the road is not firm enough to avoid this issue.