Florida to Upstate NY (Saratoga Springs)

I wouldn't expect a 30% decrease in average fuel mileage, assuming your speed is about the same in either case. Don't forget to factor in tolls as well: none on the I-81 route, some on the other route depending on exactly how you go. (The little bit of NY thruway on I-90 between I-88 and I-87 is free if you go between those two exits specifically.)

Personally, especially with a motorhome, I'd take the I-81 route pretty much every time--and indeed I have done so in a car several times as my dear mother now lives in Florida.