In an ideal world if everyone did that we wouldn't need the company to try and dictate things like this.
But even if no one abused it, the company would still try to find a way to screw people out of their personal time.
It is never going to be possible to come up with a consistent, one-size-fits-all rule on this subject. There will always be exceptions or unique circumstances for different routes. What is reasonable to expect for a driver who has an urban route with multiple options for meals and bathroom breaks might not be reasonable for a guy who is out in the country and might need to drive a considerable distance in order to find a suitable place to take a meal break.
The company has
every right to expect the driver to minimize the number of miles driven and to accurately record lunches and breaks.
The employee has
every right to take his/her lunches and breaks at a
suitable location, which in my opinion means someplace with a public restroom and heat during the winter. If no heat is available, an employee who eats his/her lunch while sitting in the package car has every right to idle the engine long enough to keep the heater in the car working.
If the company wants to take a hard line by forbidding any breaking of trace on the clock, then it should be required to rent porta-potties and position them along the route they are expecting the driver to follow.
If the employee is driving the vehicle, he/she is on the clock for purposes of insurance and liability.