I have seen the fans mounted both top or bottom of the flue - the important thing is that the fan must be pulling or pushing (as appropriate) the air up the flue since heat naturally rises. I added a five inch 'computer fan' to the top on my old coach where the refrigerator was in the slide (top vent venting to the side) and it helped tremendously.
All this said, be sure that insulation or other materials have not fallen in such a way as to block the air flow. On mine a bunch of insulation had fallen and created a partial blockage.
All this said, be sure that insulation or other materials have not fallen in such a way as to block the air flow. On mine a bunch of insulation had fallen and created a partial blockage.