I've got a suspicion the two weights you posted are the chassis specifications weights rather than the actual weights of the motorhome. Not to get off topic, but be sure to check the carrying capacity of the 24V on teh 11,500 lb chassis. You may find it nearly overloaded while the 24F on the 14,050 chassis has plenty of carrying capacity.
Now back to your questions, the fuel mileage is more affected by wind resistance, and since the two chassis probably have very similiar frontal areas, I'll be they get very close to the same mileage. Slight differenece could be in the fact that the 14K chassis might have higher numerical gearing in the rear axle which will cause the engine to turn faster.
All things considered, I bet less an 1 mpg difference.
Joe & Shelly, Justin, Tyler, Alyssa | Butler PA
2008 Tiffin Allegro Bus 43QRP|Cummins 425|Honda CRV