How can I convince my MIL that nothing is wrong with her?

It sounds as though she might be depressed and or anxious. Is she being treated for either?

Instead of trying to convince her that there is nothing wrong with her, try saying sympathetically, "Unfortunately that isn't something medical science can treat at this time. Do you think a nice gentle rub with lotion would make you feel better?" (Or a soak in a tub with bath salts, or a nap under a fluffy comforter, or a bowl of ice cream with caramel sauce ... or anything you could offer that might make her happier for at least a short time.)

What she is feeling and experiencing is real to her, even if there is no medical evidence that something is wrong. Treat her feelings with respect and offer what comfort you can.

My mother is constantly complaining she's unwell...it's a mistake when someone asks her how are you feeling, but at nearly 93 she takes a blood pressure pill, thyroid pill and cholesterol pill....nothing else..no heart attack ,cancer , stroke , ...only arthritis and mine is way worse than hers....I need my knee replaced..both her knees are her own..when we see people in wheelchairs, with oxygen masks and much younger than her I remind her how lucky she is and to stop complaining, she has also outlived a 51 year old daughter who died of cancer...

Nothing you can do. Just listen for the umpteenth time, keep your mouth shut and say "if you feel that bad, maybe you should go to hospital since dr can't find anything", get your things I'll drive you" -- that shuts my mom up pronto, no way she wants to go to the hospital.