I'm a frequent SWA flier and often fly from the east coast to the west coast. I hope you don't mind the length of the post, but I hope to share some of my experiences, and also hope to share some tips!
Previous posters are correct - SWA does not offer food on their flights other than pretzels, cookies and peanuts. Hardly something that will last you for so many hours.
You have some options.
1. Bring an insulated lunchbag as a carry on with sandwiches, raw veggies or other appropriate foods with you. Make sure it's TSA safe - salad dressing, jelly, pudding, etc., should be left at home. Do not bring drinks, even sealed water bottles - they will get confiscated. Trust me.
I bring my own lunch all the time and it works out great. I also bring an EMPTY filter bottle and fill it with water at the airport after the TSA fiasco, er, search. It saves me about $4 a bottle on water. If you choose to do this, make sure you fully loosen (almost open) the top of your bottle during the flight. You see, the air pressure will squeeze the water OUT of the bottle's filter straw during the flight and your lap / purse / carryon / seatback pocket will get wet. Again, learn from my mistakes! LOL
2. You can always purchase food for the trip at your departure airport, after you pass security. There are many restaurants and delis that will pack stuff up to-go. Be aware, however, that they know they have a captive audience and will charge an arm and a leg for the convenience. For example, the last time I was in the Las Vegas Airport, at Burger King, a simple Whopper meal cost nearly $15. Same in California, Phoenix, Nashville, etc.
3. Depending on how long your layover in Phoenix is, you might be able to sneak out into the airport and get some fast food, though there's not an awful lot of the fast food variety at PHX (I was just there 2 weeks ago), at least not in the terminals I was in. Your crew may not allow you to exit though, so that's more of a chancy thing.
Personally, I'd pack my favorite foods the night before, pop it in a soft insulated bag, and keep it in my purse or other carryon. Some good foods that travel well are carrot and celery sticks, nuts, sandwiches on hardy bread (ciabatta rolls are excellent for this, but refrain from using mayo as that could spoil), apple slices (spritz them with lemon juice to keep them from browning), raisins, dried fruits, beef jerky, lunchables (careful that they don't have liquid in them!!), and cheeses (remember the funk factor, though). Avoid items like tuna fish or other "smelly" foods (no limburger!!!) that could get you a lot of dirty looks and maybe a mean comment or two.
If you absolutely need mayo on your sandwiches, go for the little packages that are found at many sandwich joints.
I've brought my own foods on SWA flights quite a few times. Not only does it give me the freedom to eat when I need and want to, but I can stay on any dietary restrictions. Moreover, I'm saving a bunch of money this way, rather than selling my firstborn for a lukewarm hamburger.
I wish you luck and a delightful trip!