I would suggest a bit of transposition and italicization, as follows:
"I am going to the store. I will probably buy some food. It's getting late." Isn't it reasonable to assume that Bob might have said that?
Or, it could be kept as a single sentence:
"I am going to the store. I will probably buy some food. It's getting late-" isn't it reasonable to assume that Bob might have said that?
Or, (colon optional)
Bob might have said: "I am going to the store. I will probably buy some food. It's getting late." Isn't it reasonable to assume that he might have said so?
As for the sentence you're using right now, the question mark would be better placed outside of the quotation marks, as Bob's final statement isn't an interrogative one. I'd also suggest italicizing it, but it's still not particularly elegant.
Isn't it reasonable to assume that Bob might have said, "I am doing to the store. I will probably buy some food. It's getting late"?
I saw in the comments that you'd consider putting the question mark outside of the quote if the Chicago Manual of Style said to do so. In a forum post, they write:
The only end punctuation that would go outside the quotation marks would be a question mark or exclamation point (unless it was part of the quoted material).
Again, the question mark is not part of the quoted material, so you ought to put it outside of the quote marks.