Sorry you are having such a difficult time. Standing on your feet all that time can make your body go haywire, especially your spine, feet, and stomach muscles. I would suggest seeing a orthopedic doctor to begin with. I can also see where your weight issues come from. First, you walk around for hours a day, bending, lifting, carrying food around is just like dumbbells. You are expending more calories than you are consuming. From your explanation of your life, it sounds like you only eat once a day. You can't expect to maintain your weight, much less gain any if you don't consume more calories.
Your current weight is getting to that dangerously low level, where it can effect other organs. The fact that you sleep almost all the time you aren't working, says that your body doesn't have any spare energy(calories) to expend. Working or sleeping all the time is no way to have a life. You are so lucky to have a great family. You need to make an effort to consume more calories whenever you can, so you can have more energy to spend with your family. Sneak in weight gain shakes, with protein, whenever you get the chance, while in the car, on a break, or even have one open and ready to take a swig between customers or picking up orders. You can find them easily at your local grocery store, even at Wal-Mart. You will probably feel better almost immediately. You still have to make an effort to see a doctor about your pain problems.
Good luck to you, and let us know how you are doing. We care.