Seems like you are pursuing two inquiries: Why do people with ALS experience global fatigue, and why do you feel progressive fatigue during the day?
I don't really know the answer to your first question, but I have been told by at least one well educated person that the answer isn't clear. One common thought is exactly what you stated, that the gradual weakneing of muscles requires the body to work much harder.
Your second question isn't necessarily easier to answer because it depends on what is ailing you. There are a number of neuromuscular related illnesses (like myasthenia gravis) that improve with rest, and it is a good sign what you feel is not ALS.
The days of , "I'm tired , but I just keep going" are done.
I treat my energy like a gas tank .. when its getting empty , I feel tired , the leg flops more , I start to yawn. Napping helps, taking regular vitamins, and a good diet helps. I find snacks , like fruit etc will carry me through.
I am still working daily , though its a desk job. I have learned to delegate and get people to come to me in order to keep the energy up.