I checked into the same thing.  I looked into a 10,800 watt generator.  The salesman told me that in would require a 150 gallon propane tank at 60 degree ambient temperature to suffice the pressure needed by the generator.  Now on the back of my food truck I planned on only having 1- 100 lb propane tank.  1- 100lb propane tank only holds about 24 gallons of propane.  So even if I put 2- 100 lb propane tanks I wouldnt be able to suffice the gallons required by the generator.  This generator also used 2.2 gallons of propane/hr.  So in a 5 hr shift the generator alone would use 11 gallons of propane at about $2.40/gallon= $26.40.  A good gas generator will use 1.125 gal of gas/hr.  So with some math=  5hr*1.125= 5.625 gallons of gas in the same 5 hr shift.  Here gas is about 3.46/gal *5.625gal= $19.46.  So it would cost about $7.00 more per 5 hr shift.  5 days a week, 30 weeks a year==  $1050 more in fuel cost per season.  These numbers are in my area and how often I will run my food truck.  Now you still have to suffice the gallon requirement.  You need 150 gallons at 60 degrees, which means about 6-100 lb tanks! Just for the generator!  I still have my cooking equipment to run.  The colder it gets the more you need.  At 30 degrees I would have needed 250 gallons.
 
Now this is the research I did based on my needs.  All of this was a little out of my element. And I hope I researched it appropriately and I didnt just blow a bunch of smoke up you know where.  Hopefully someone with more expertise in this will confirm what I have said.