I have 300+hrs on my dk45 se hst which is almost 2 years old.  When cutting grass at high rpms I use a good bit of fuel.  When doing FEL work and not travelling at high speeds I don't use nearly as much.
An additional note when comparing the DK's fuel consumption vs. other models/brands, take into account the weight of the two different tractors.  More weight requires more power to move.  I start my tractor at lower rpm's then crank it up as needed.  I tramp down on the hst pedal everytime I am on the road, or mowing grass.  I don't cut grass in high and medium seems too slow (HST).One other thing to consider, take your loader arms off your tractor when cutting grass.  You will notice a very big fuel saving when your not lugging about several hundred pounds.  If taking the arms off is a bit of a chore, make sure to remove the bucket at least.
My 45 does not have a 11+ gallon tank.  No way it does.  When I bought it the website said it did.  I have filled that tank so many times I know it takes about 8 when it is almost bone dry.