Totally agree that dry mouth is not a symptom of any STD. I can't tell you what is causing your dry mouth, but it isn't an STD.
I'm sorry you've been through what you've been through. :(
Your symptom is dry mouth, correct? I see that you were already advised that you had no risk for HIV. So, please do not try to correlate the dry mouth to that as it would not be the case. Zero risk means you can rule that out. Other std's? Well, not on the list of things that cause dry mouth. Here's a list of possibilities for you to look into https://www.mayoclinic.org/diseases-conditions/dry-mouth/symptoms-causes/syc-20356048 (copy and paste to your browser). Do you use tobacco or drink alcohol? Do you take any medications? Look at this link and see if anything matches. STD's and HIV are not the cause. If you have an overgrowth of yeast, they can tell you very easily. See your doctor.