If she has been on that hot feed since April and yet she is still losing weight than I certainly think you need to have her checked out more thoroughly. A 10-15% loss of body weight is pretty profound.
If EPM is a problem in central Texas than it certainly should be considered as a possibility, but there are many other causes for chronic weight loss. Diagnosing EPM can be difficult because the most available tests only tell you if the horse has been exposed to the disease, not if they are actually suffering from it. That being said, I think a serum test is warranted.
Internal parasites (worms) are extremely common; however, it sounds like you have a good and frequent deworming program so that is less likely.
Likewise, bad teeth that either hurt her mouth or do not allow her to chew and swallow her food effectively are a common problem, but it sounds like your mare's teeth were just checked and floated in April. Have them checked again when you can, just to be certain.
A chronic smoldering infection that is difficult to detect is a possibility, in which case a blood count may be helpful as would antibiotics.
Stomach ulcers are a possibility, and hot feed can be associated with them.
Cancer is always a possibility that has to be considered in chronic weight loss cases.
Nutritional deficiencies may also be an issue. Even though you are giving some pretty hot feed, certain vitamins, such as vitamin E, may be deficient or not present in enough quantity. Vitamin E deficiency is an antioxidant that offsets the damage caused by oxygen free radicals. Deficiency causes atrophy of certain types of muscle fibers (called Type I muscle fibers) and deficiency can result in muscle atrophy, weight loss, difficulty standing and even muscle twitching. Vitamin E supplements can be useful in treatment.
I know I just threw a whole lot of information at you but I want you to be well prepared for discussing your next move with your veterinarian. Because she has lost all that weight, I really think a more extensive work-up by your veterinarian is warrented. Please let me know if I have been helpful and also what you discover with your mare. I am really hoping it is something common and easily treated.
p.s. I have been around Arabians all my life, in particular Crabbet Arabians from the Indian Magic bloodline.