can I go to the U.S on a tourist visa and then apply for a fiancee visa from the U.S?

As I understand the question, the US citizen is returning from France, and the non-US party wishes to enter the US on a tourist visa and then apply for a fiancée visa from the US. Assuming that I have the fact situation straight, a fiancée petition contemplates the non-US party being overseas and interviewing at an American consulate or embassy before entering the US. If your idea is to have the non-US party come with a tourist visa and then change status to a fiancée petition while here in the US, that cannot be done. Due to the limitations of the Lawyers.com Forums, Alan Lee, Esq.'s (the "Firm") participation in responding to questions posted herein does not constitute legal advice, nor legal representation of the person or entity posting a question. No Attorney/Client relationship is or shall be construed to be created hereby. The information provided herein by the Firm is general, and requires that the poster obtain specific legal advice from an attorney. The poster shall not rely upon the information provided herein as legal advice nor as the basis for making any decisions of legal consequence.

You cannot apply for a fiance visa yourself.  Only a US citizen can petition for his or her fiance.