When you run out of ammo, fuel or food your only option is to rush to a city. But what if that city is too far away or your fuel runs out and you cant move it to a city area? So i suggest adding two types of units;
Supply trucks; they can fully supply units with any type of resource. They can hold 750 ammo, 750 fuel and 750 food.
Supply planes; they drop supplies onto land and sea units and are alot faster than supply trucks, but they carry less resources. They can hold 500 ammo, 500 fuel and 500 food.