I don't have a MH, but here is how I'd look at it:
I assume the Jayco is bigger than your old one? If so, item #1 that works against mileage - I assume the size it what you want and wor the sacrifice in mileage?
How many miles do you expect each year? I'd figure fuel cost at $4 times the mileage you travel and compare that cost against the convenience of the new rig. Unless you drive crazy miles, the fuel cost may not be all that significant. Don't forget the crazy high oil change costs on a diesel too.
2016 Jayco Eagle HT 29.5BHDS (ordered 12/30/15, delivered 3/8/16)
2015 F-350 crew cab, short bed, 6.7L PSD, Pullrite Superglide 3300 hitch