Fuel Tank Size?

[Bucket2]

Can someone tell me why my new (Used) 06 26SS Greyhawk Class C which I have reason to believe has a 57 gal (55) usable tank will only hold 37-40 gallons of fuel when the fuel gauge registers very near empty?
I seem to remember the service man at the dealership was trying to explain something about this to me but obviously didn't listen well. I can get back in touch with him, but just thought there would be more reliable information on this forum.
Thanks for any help you have
Bucket2

[BullDozerBob]

Gauge is off most likely. My Greyhawk never got any better than 7-8 miles per gallon. Every 200 miles I knew half of the tank was gone and looked for someplace to fuel it. When fueling the drivers side needed to higher than the passenger side on the ramp or I could only get the tank 3/4 full. On a 2004 Jayco 25DS the tank and the spout are almost on the same plane.

BullDozerBob

Spokane Wa

[Bucket2]

Thanks for taking the time to reply.
Those are the two possible answers I thought of but was hoping for some little quirk special to Jayco. It just seems like a nuisance to either leave 20 gallons in the tank of not fill it the other twenty. It would be pretty easy to test the sending unit if it comes out the top of the tank without lowering it, I cant think it would be the gauge itself. Either way I appreciate you taking the time.
Bucket2