Why does my food and fuel supply always drop whenever I reach 500,000?

I don't really have an answer for that but based from my experience, this means you have more troops that are needed to be fed. Troop consumption over food production, your troops eat more than what you can produce. What I did was lessen my troops, which means filling up my hostipals first then killing them. Train only what your apc can handle, you can train more but not too much.