I agree with user RatRacer in that if you paid approx. $5500 for that ZTR you got a good deal. But, my research on the Husqvarna website yielded this model(actually a P-ZT4822FS / 966613301 according to the manual I found) being sold in 2010-2011, so I don't think you got a 2012 model like you were told:0( How many "hours" does it show on the dashboard???
That being said, it shows an 11gal tank, so I would have to ask if you are 101% certain that that particular tank was COMPLETELY full when you started??? And....how much "area" did you cut before it ran out??? That Kawasaki FX651V is "very similar" to the FX691V that is on my Gravely(22hp...mine is 726CC and I don't see the actual "size" in your manual). I have been tracking my fuel usage over the last dozen mowings and I am mowing approx. 1.5 acres on exactly 1 gallon of "non-ethanol 89 octane" gasoline. I would HIGHLY recommend, if it is available in your area, that you NOT use ethanol gasoline in that ZTR and run the mid-grade(89 octane) non-ethanol gasoline ONLY!!! I even add StaBil gas treatment to mine in every tankful just to be extra cautious. If this ZTR has been using ethanol gasoline I'll bet a dime to a dollar that is contributing to the bad fuel economy and I'm surprised that if stored with gas in it for the length of time that dealer has had it....that it hasn't begun to deteriorate the fuel lines and the Kawa's carb. If I were you I would get some Sea-Foam(available at your local auto parts or Walmart) and add the recommended amount times two in a fresh tank of "non-ethanol" gas and run that through it. If the other tank still has gas in it , again add a double treatment of Sea-Foam to it before running it. And then start using non-ethanol only. I'll bet the fuel economy will return to "normal" then(like me, I would expect you to get AT LEAST an acre per gallon). If I were you, I would keep that ZTR for that money and follow the gasoline advice I gave you. If you take care of it and stop running ethanol gas I think you will be pleased with that purchase(and the reliability and longevity of that "commercial" Kawasaki engine)!!!
P.S.
Here's a link to that online manual if you will find that useful:
http://www.husqvarna.com/ddoc/HUSO/HUSO2011_NAen/HUSO2011_NAen_PZ-TOM_115392427-01.pdf