Check with your physician or pharmacist if your prescriptions, as well as nonprescription items, are heat or cold vulnerable. Rotate your food supplies. I recommend hiking bars you can eat and replace frequently. MREs, nuts, high calorie bars, etc. are also good. Rotate batteries more frequently.
Water is heavy and bulky.
It will constitute the largest amount of weight allocated to supplies. Check into filtration. Filter straws are available that can generate 10 gallons of drinkable water from a mud puddle. For convenience, you can store parts of your kit in different areas of your automobile. Food can be protected in the trunk in an ammo can or Tupperware container.