sounds like you have had a pretty significant work up ... your imaging will pretty much r/o MS, any vascular lesion although gado with the MRI or contrast with the CT would help, mass, clot, brain stem lesion, middle/inner ear mass/lesion/hemorrhage; and I can imagine that they worked you crazy for any possible vasculitis(ESR)/neuropathic(like lyme/B12/TSH/hgba1c/SPEP/amyloid)/infectious(syphillis) process ...
do you have headaches associated with the vertiginous feelings(did your neurologist think any chance that you have some type of basilar migraine) ... do you notice the sensation only when you position your head in any specific direction(suggestive of some positional vascular compression, have they done CT/CTA of posterior circulation or ultrasound of you neck/posterior circ)
other ?'s .. sounds like this is real "vertigo and not "syncope" .. you don't have "fainting" spells, no reported seizure like activity ... any other health issues or meds that could be contributing ...
cause if your work up has been as extensive as it sounds, I expect you be "slapped" with the diagnosis of idiopathic vertigo; the only thing I can suggest is that you go(if you already haven't) to a larger academic center where you may find a "world's expert" ENT/neurologist on vertigo
best of luck ...