Personally, I don't think a money lawsuit is the answer for everything(you haven't said that that is what you're going to do, but people seem to be suggesting it). Every lawsuit just propagates the next and the people who get hurt the most are the employees who may be laid off when a business has to downsize to help pay judgements.
I've always thought that if something was bad enough to sue for, then jail time for the specific offender, not punishing everyone by bankrupting the company, would be the best justice in many cases.
Now, jail time in this case might be a bit harsh; but instead a good start would be a statement of what's happened and a copy of the coupon, sent to your local news stations. Can't you see it now? "News at 11- Can't park in Handicapped spot, Here's a free sandwich". Once they see that, that will probably increase their response by several magnitudes...
Ultimately, one employee made a poor judgement but their home office has apparently decided to compound it with apathy. It's unfortunate that you now have to take it to the next level in order to embarass them into taking more appropriate action, it seems like no company wants to take responsibility for things unless they are either sued or embarrassed in the news. At a bare minimum; I would have thought they would have: A. Given somewhat more free food than one sandwich B. Acknowledged the error in writing and C. Designed and presented you with an official Sonic parking tag signed by the highest corporate officer that entitles the bearer to park in any Sonic parking spot without harrassment.