Of course, the most common way to reduce your food expenses for the trip is to brown bag it. Take some sandwiches, chips, snacks, or whatever they like to eat. If you are brown bagging it, make sure that what ever you are putting into this bag is at least semi-non-perishables. Meaning, I'm sure would not be a package of fish in the bag. Take food with you is one of the most common ways to save money while traveling or brown bagging it at work to save money. But as I said, I would not recommend fish or something similar if they have a refrigerator. The types of odors persist, and the money saved by brown bagging only be absorbed by the cost of air fresheners!
Bring a cooler
Enjoy the sound of muddy ice in the refrigerator while traveling. You can fit many different things in an ice box, depending on their size and the amount of space you have available to take with you. If you are going to be on a long trip and want to save money, I recommend you make some space for the refrigerator. You can pack sausages, cold drinks, and more in a cooler. Instead of spending a few dollars on a drink while you're away, you can get back in your cooler and have a drink brought from home that is at a fraction of the cost of petrol.
Search the Internet for restaurants along the route of travel. Some may have websites that you can print coupons. Save 15% or being treated as a 'buy one get one free' is a great way to save on food during the trip. Also give you the opportunity to get out and stretch our legs and relax a while before going out on your trip.
If you want to save on food while traveling in an airplane your options are somewhat limited in their personal vehicle travel. You need to check the airline rules and regulations on the types of foods and drinks are available onboard. Usually are not. However, you could always paper bag and leave the rest of the luggage. Thus once you pick up your luggage you can take a bit of food that brought that instead of spending so much money at the airport. Also, check your flights and see if there is any discount on meals, etc. If your trip is not going to take time to eat at home before leaving.
Cook your own food
Buying food at the supermarket and cook for yourself is usually cheaper than eating in restaurants. You can stay at extended stay hotels or hostels - usually have cooking facilities. As a bonus, you also save money staying in extended stay hotels compared to regular hotels or hostels.
You have many options of how to save on food during the trip. The easiest way to save money on food is when you are traveling in your own car. When you travel in this way have more space and the ability to effectively carry food and drink with you. If you have the space, even for a small ice cooler in your vehicle, which will save several dollars on soft drinks at rest stops and service stations.