I'm willing to bet your problem lies in that little dial you're using to measure temps and humidity. Digital or analog isn't the issue here, the issue is that is measuring air temps. Air temp only matters at the coolest part of the cage, and is never to drop below 75. What you need is a Non Contact Thermometer (Temp Gun) to measure the surface temps on the floor of both the hot side and the cool side. Other than that your setup looks excellent. Does she spend alot of time on the cool side? because if you're getting a reading of over 90 degrees on the hot side with that dial, the surface temp on the floor has to be far greater than 90 degrees and I imagine she probably stays off it most of the time. The air on my hot sides are probably closer to the high 70's, low 80's, while that basking spot on the floor where the animal absorbs heat through it's belly is exactly 90 degrees.
So first off, get a good temp gun and determine what exactly is going on as far as surface temps on the floor. As far as treating her, what I'm going to suggest is first push down slightly on the bridge of his nose and check for any bubbles in the nostrils, or any fluids at the corners of the mouth, if you see either of these you have a URI that will need to be kept a bit drier for now, and the temps a bit warmer.
If there are no signs of URI then try getting a rubbermaid tote similar to your enclosures, fill it about half way with 90 degree water (make sure it isn't too hot, our body temp is 98.5 so 90 degree water may not feel as warm as you would think), and put him in it, put the lid on, and let him sit for about an hour. The temperature will boost his immune system slightly, and the humidity will loosen up anything in it's nostrils or lungs such as retained shed or small amounts of mucus.
Then just verify what your floor temps are. If you don't have access to a temp gun just yet, you can temporarily use an indoor/outdoor thermometer that has a wired probe to measure outside temps, and use that probe taped to the bottom of the tote, one on each side. Again, remember, anytime any are of the cage drops below 75, you are asking for a resporatory infection.