Unless you've already ruled them out, you might consider food sensitivities. They can cause weight gain without other obvious symptoms.
A friend of mine was always heavy when we were growing up, she tried so hard not to eat much but was always the "fat" girl. When we were teenagers she was finally told she had a milk sensitivity and she cut out dairy products made with cow's milk. She lost 40 lbs in a year without any other diet changes.
You mention that you give her jello, and I think you mean "sugar free" since jello is fat free as far as I know. Anyway, I've heard recently that some people have adverse reactions to the artificial sweeteners. If removing dairy doesn't seem to give any results then try taking those out of her diet, let her have a little sugar instead, you have nothing to lose if you try for just a week or so.
The fact that her weight can fluctuate on a daily basis leads me to believe you are correct that she has bloating/water weight issues. SOMETHING is causing that and sine she's been tested for endicrine issues then I think it's likely that she is ingesting whatever is causing the problem.
Gee, Renee. Thanks. I guess you missed everything I wrote about how she doesn't eat, waht she will eat, and the fact that she is already very active. Did you actually read my comments? I saw a part of this answer, verbatim, on another posting. I guess one-size-fits all in your world.
Suggest cooking and giving only low fat meats (chicken, fish, loin of pork) or non-meat sources (soy products, nuts), low fat vegetarian combinations (rice & beans, pasta & beans, low fat cheese & pasta), fat free or low fat dairy products (skim milk, 1% milk, low fat cheese, low fat yogurts, low fat soy milk/yogurt), use plant oils, non trans-fat margarines, and eat a lot of fruits and vegetables. Eat daily high fiber breads and cereals (oatmeal) and no ‘junk’ foods or snacks. Have your child go and play outdoors, run, and walk too. Limit time in front of the TV or non-active-video games. There have been studies showing the longer the length of time a child sit in front of the TV, the greater the weight gain. Hope this helped you.