I'm a guy and pretty soon, I'll be meeting my family-in-law for the first time. As you probably can imagine, I'm a bit nervous for this encounter, also because I'm not exactly the perfect son-in-law. I happen to be in a wheelchair, and I'm wary as to how they'll react. Of course, my girlfriend has told them she's dating a disabled guy, but still...

Or maybe avoid dating period. You might still have to spend a lot of time with the parents if your SO is a very family-oriented person.
Just remember nobody's perfect and you are her choice not theirs.
However, as stated above, chances are your girlfriend isn't exactly foreign in her acceptance of others so if there are family members that shun you due to your disability, likely she'll deal with them so you don't have to and she will feel enough disgust to justify shunning them.