Hope you still are checking for replies. I was discharged from the the USAF (1972-1976) and worked on radar jamming equipment (ECM for electronic counter measures). I am now a retired RN. I had similar feelings about veteran disability benefits. I didn't decide to apply but was TOLD to apply and it seemed more like an order. I had a knee injury and had surgery while on active duty. I was grateful for the care I received and never felt disabled afterwards. I applied and expected a 0% rating at best but if denied that would not have bothered me. I ended up being rated 30% which at the time was probably $75 per month. I was handy while working toward my a BS degree - not in nursing but something else -- nursing came later. I paid taxes and considered it a tax rebate.
I haven't been diagnosed with AS but the symptoms are "AS like" but actually something else I guess. Anyway, my take on getting approved for VA disability so long after the fact --- probably difficult. AS is largely heredity and therefore you were prone to it... you would have to prove that military service caused it or made it worse. I think there are valid reasons for why military service may have triggered it but hard to prove because nobody seems to know exactly why it occurs except it seems largely genetic. That's just my opinion.
If the condition was diagnosed in the military, and you were being treated for it in the military, and your military records document it OR certain military activities made it worse and that was documented than maybe there is a good chance. The "upset stomach" part you mentioned makes a case for a related disease called reactive arthritis which apparently is very similar to AS. It seems like it may depend on who makes the diagnosis.
I say -go for it! Good luck with nursing school. The process of applying and exposure to the VA way will be a useful medical experience. Immune system diseases are only beginning to be understood. Keep us posted.