Are you talking about a long-term "it's better for your health if you eat more fiber and lower your sodium" situation or a "you have a medical condition that is triggered immediately by high sodium in your blood" situation?
If you are in the first situation, ask your doctor how much you can 'cheat' during the trip. If you are in the second situation, ask the doctor if there are any meds you can up/change to make it a little easier to be off a bit on your trip.
You can get plain raw vegetables in the food courts at the resorts, and lots of fresh plain fruit all over the parks. At the restaurants ask for plain grilled chicken, salads with no dressing/no processed additives, etc. The QS places have salads with dressing in packets - so you can try to find a low sodium one, or skip the dressing all together.