Soft ground after October rains: will the service truck sink?

We've got a commercial site with 40 workers, running for 12 weeks. I ordered four standard units and one ADA-compliant unit to meet OSHA 1926.51. The access road is dirt and got soaked in the October rains, it's muddy and soft now. The service truck needs to get within 25 feet for weekly pumping. I'm worried the vacuum truck will sink or get stuck in the mud. What's the best way to handle this? Should I ask the rental company for a lighter truck or put down gravel before the next service?