What makes you think I have not? In fact, I became a vegetarian at age 17, and I started growing all my own organic food at age 20. I live in the mountains of BC, and I drink clear mountain water. I have never smoked, and in fact, I have crusaded against smoking in public places. I do use vitamin and mineral supplements, because my body does not digest food well. So I don't get infections, which most people who are on biologics get. I ground my own grain and baked all my own breads, created all my foods from scratch. I also did gluten free and vegan, long before they became the popular fad they are today. But despite living a very clean lifestyle, and never eating junk food or sugar, I still got RA. Because RA is a genetic illness, and the cure does not lie in lifestyle changes. However, I do believe that people need to eat better, exercise more and live for God.
But once an auto-immune disease gene is turned on, there is no way to turn it off. Scientists are working on that as a "cure" for auto-immune diseases. But who knows how long that will take? God did not create RA. It is a result of the Fall. Sin and death came into the world, when Adam and Eve fell in the Garden of Eden. Man and woman were created perfectly. But because of various environmental factors which came into play after the fall, the genetic load is increasing everywhere. Genes have become mutate, and auto-immune diseases are just part of that. Fortunately, Jesus is going to be returning before things get so bad that no one can survive in this fallen, sin stained and damaged world.
I am grateful that God has used my sickness to develop my character in him. Suffering does create character.
"More than that, we rejoice in our suffering, knowing that suffering produces endurance, and endurance produces character and character produces hope, and hope does not put us to shame, because God's love has been poured out by the Holy Spirit who was given to us." Romans 5:3-5
God is in control, and the answer to the Question asked in the OP title - no, it is not a sin to take medication, whether for mental or physical illnesses.