Did the Prophet Muhammad (peace be upon him) fast the entirety of Ramadan during campaigns and battles, or did he fast in some and not in others, or did he never fast during battles?

The Prophet, peace and blessings be upon him, did not engage in battle in Ramadan except while traveling, and that was in Badr and the conquest of Mecca. It is narrated in Sahih Muslim from Abu Sa'id al-Khudri that they traveled with the Messenger of Allah to Mecca while they were fasting. He said to them: "Indeed, you have drawn near to your enemy, and breaking your fast is stronger for you." So it was a concession, and some of them fasted and some broke their fast. Then he said to them at another stop: "Indeed, you will face your enemy in the morning, and breaking your fast is stronger for you, so break your fast," and it was a firm command, so they broke their fast.