Keep in mind, you may have been Dx'ed with MS but that doesn't prevent anyone from developing other age related"
Think of all the TV ads you see for eye related OTC drugs IE: Visine.
I personally have never heard of dry - eyes related to MS........but. How about other drugs you take regularly. I could see that being a common problem.
Probably the biggest one commonly used would be allergy meds. Particularly antihistamines. This is a very common side effect of many of those meds.