Is it possible for an organism that is polypoloid with an odd number of sets of chromosomes (e.g. 5 sets) to be fertile?
Yes, it is possible. There seems however to always come up with a cost in fertility eventually of the order of 2/3!

You should have a look at Ramsey and Schemske 1998. I don't remember it well enough to make a good summary (maybe someone else could do that?!) but it is a classical article. Here is a sentence from their abstract


  In contrast to the common claim that triploids are sterile, our results indicate that the triploid bridge pathway can contribute significantly to autopolyploid formation regardless of the mating system, and to allopolyploid formation in outcrossing taxa.