From IRS Publication 463:
Travel expenses defined. For tax purposes, travel expenses are the ordinary and necessary expenses of traveling away from home for your business, profession, or job.
An ordinary expense is one that is common and accepted in your trade or business. A necessary expense is one that is helpful and appropriate for your business. An expense does not have to be required to be considered necessary.
You will find examples of deductible travel expenses in Table 1-1 , later.
Traveling Away From Home
You are traveling away from home if:
Your duties require you to be away from the general area of your tax home (defined later) substantially longer than an ordinary day's work, and
You need to sleep or rest to meet the demands of your work while away from home.
This rest requirement is not satisfied by merely napping in your car. You DO NOT have to be away from your tax home for a whole day or from dusk to dawn as long as your relief from duty is long enough to get necessary sleep or rest.
I think a 10- to 14-hour day is substantially longer than an ordinary day's work. I think it's reasonable to assume that most people, unless they are superhuman, cannot work and drive for that amount of time without a significant break and still adequately perform their duties. I know I can't.
I would say that people need to eat, as well as take a break, during a day that long. There's nothing that says or indicates they have to brown bag it or pack a cooler with food.
Under the specific heading of "Meals," the IRS again states:
Meals
You can deduct the cost of meals in either of the following situations.
It is necessary for you to stop for substantial sleep or rest to properly perform your duties while traveling away from home on business.
or
The meal is business-related entertainment.
Maybe many mystery shoppers can indeed be on the road all day and not have to rest or eat. Maybe they don't have to go to the bathroom, either!
But that doesn't mean that everybody can do it, or that they should, or that they need to. It's not some badge of honor to work for 12 hours straight without needing a significant rest or needing to eat.
From the further reading I just did, it seems that some self-employed people don't have an established "tax home" (as defined by the IRS). In that case, they might be considered "itinerants" and can't count meals on the road as an expense. It could be that category might apply to some shoppers who only do out-of-town routes,
I learn something new every day, but not everyday!
I've learned to never trust spell-check or my phone's auto-fill feature.