Service truck got stuck in our muddy field after October rains. Who pays for the tow?

Check your rental agreement first, because the access clause is what decides this, not who had the worse day. Most agreements I've signed put the customer on the hook for keeping a clear, drivable path to the unit, and the truck access section usually says the driver can refuse or reschedule if the ground looks bad.

That last part is your leverage. You told them the field was soft and the driver still drove in. The driver is the one who decides whether to leave the gravel, and a pro who sees axle deep mud coming should stop at the road and call it in. I'd write back, quote whatever warning you gave (text, email, call log with the time), and ask them to split it or drop it.

For the rest of the season, fix the root problem:

• Move the unit within a short hose reach of the gravel road. The vacuum truck hose runs a long way, so the truck never needs to touch the pasture.
• Set it on a level spot that drains.
• Tell dispatch in writing to stay on the gravel after rain.

We did exactly that on a job with a soggy back lot and never had another stuck truck.