I agree that the heat can make MS so much worse. In researching your question I have found several links which may be helpful.
According to an article entitled, "Tips to Survive the Summer with Multiple Sclerosis" by Julie Stachowiak, Ph.D "If an air conditioner is needed for your home because of MS-related heat intolerance, the cost of this equipment may be tax-deductible if your doctor has written a prescription for it."
So it seems you can get a tax break on the cost of the air conditioning unit itself.
This is also consistent with what the National MS Society states on their pages devoted to giving tips about the heat.
As far as getting assistance on the actual cooling bills, this I do not know. It may vary from state to state whether or not assistance is granted.
This is an excellent question to ask your MS local chapter and here is the telephone number to do just that:
I hope this helps some! Thank you for your question.