Actually your 2000 D is 37' 9" long with a base weight of 17,000 lb.
You didn't say what speed you drive at.
The 11 + MPG you got with the last MH was very unusual if it was the 24 Valve ISB.
Usually only some of the older 12 valve 5.9L would attain that.
My Average over 8 years with my ISB is 10.22 MPG speed around 55-65 MPH.
If I keep it at 55-58 average will go to around 10.7 MPG.
Usually pulling a 3,300 lb toad. GCWR around 26,200 lb.
But even when taking long trips without toad I never had one tank to hit in the 11 range. Probably drive a little faster then.
I do know of one D owner with a 2000 says he gets 11.3 MPG.
But non of the other D owners even mention of being in the 11 range.
Anytime you need information with problems on your D.
Try the Discovery Owners
but it does cost to join.
My 99 has a 4.63 rear end. Mine is a Meritor RS-17-145
15,500 GAWR and yours is a 17,000??.