There's no way to tell if there are spots available in advance. But I just called this specific Cracker Barrel and spoke to a manager who said they do allow RVs to park overnight in their parking lot. She said there are RV designated spots that you'll see when pulling in.

We just called this Cracker Barrel in Ft Lauderdale and asked for permission to park our car with 15 ft camper overnight. They politely explained that they don’t allow overnight parking at all in their lot. After not getting into our campground in time and all others already closed or full, it would have been nice to have that alternative to rest up for a short bit.