This translation is actually older than the "Treasury of Scripture", as we find it already in the Jewish targumim and the works of Josephus.
However, in my opinion targum Jonathan's novel translation is a good example of violence done to the text (perhaps to vindicate the spies from seeking lodging at a prostitute's house). The Hebrew word "mazon" מזון meaning "food", is never used as a verb as is the case with English where the noun "food" changes form and becomes "feed" when used in a verb phrase. So to say that zonah here means "someone who feeds others" is virtually unsupported from the bible. Actually there is another Hebrew word for feeding others found in 2chronicles 28:15, ויאכילום. So if anything the word for innkeeper would be something which contains the root אכל not זן. As for the targum, in fact Jewish tradition itself confirms that Rahab was a prostitute, see here.
In any case, looking at Strong's concordance the word zonah never comes up in the context of innkeeping or feeding others, but it does connote lewdness, harlotry, whoredom and fornication, so it is pretty clear that zonah here means nothing other than "prostitute". According to Wikipedia, however, Rahab could have been both a prostitute and an innkeeper as it was not uncommon for an inn and brothel to operate within the same building.