NOTE: I both trucks I mentioned are 8' beds.The Tremor is only available as a crew cab 6.75'bed. I do not live in town and my truck does not have my camper on most of the time. I have more use for a truck with an 8' bed than most people.
The OP was asking about a truck for a Grandby (made for an 8' bed),
I would not recommend a short bed truck for a Grandby. yes it can be done but I have not enjoyed driving a loaded truck with the center of gravity really far back.
In states where registration cost is based on GVWR there may be more reason to get the F250 over the F350. but compared to what the truck costs does it really matter?
I do not have to be concerned with registration cost here in TN, my Insurance has been interesting because apparently I am unusual in owning a truck for personal use that most owners use commercially.
as far as MPG, you will not see an appreciable difference with the 3.73 vs 355 vs 4.30 at 80mph. You're pushing a big object through the air and it will take a lot of power to do it. If you're concerned about MPG's slow down it will help more.
My reasoning for getting the 7.3L engine and the 4.30 gears is:
1. Ive never had too much engine.
2. Ive never had too much gear
3. this may be the last truck I buy
I wasn't in any way trying to imply get a short bed, I was merely using the tremor as a known example where if you order a F250 or F350 there is no functional difference in the trucks except the badging and sticker. I was making the point that an F250 is essentially "derated" and that if you're running at 90+% of the payload on that platform you aren't actually pushing anything in terms of components. Also to point out don't be afraid of an F350 thinking its going to have a harsher ride than a F250, in cases they are sprung the same, what options you order matter more than which badge if ride quality is a concern.
I think some states can gouge alot for a 1T but that's where as a consumer you need to be aware, its not a big deal here that I am aware so I ordered an F350. As an example in the other direction my buddy in MN ordered a 1T because its actually cheaper to register than a 3/4T in that state he said. 
Caveat : The below is just my brain being nerdy, not being argumentative, I appreciate differing perspectives and do change my opinions when merited.
Off hand its hard to imagine there wouldn't be an appreciable difference between 3.73 and 4.3's at western highway speeds. With the all terrain tires that are 34.1", a good 300+ rpm difference between the two and in my mind in meaningful ways (rpms over 2000rpm). I mean if there were extra overdrive gears (in addition to the 3 already there) it would neutralize that concern with the 4.3s but as is those extra 300rpm required seem like they'll spool the engine more in a gas consuming way.
2000rpm 10th gear 3.73 = 79.3mph (~2300rpm with 4.3)
2150rpm 10th gear 3.73 = 85.2mph (~2480rpm with 4.3)
1600rpm 10th gear 3.73 = 63.4mph (~55mph with 4.3)
1700rpm 10th gear 3.73 = 67.4mph (~58.5mph with 4.3)
1800rpm 10th gear 3.73 = 71.4mph (~61.9mph with 4.3)
1900rpm 10th gear 3.73 = 75.3mph (~65.3mph with 4.3)
I acknowledge wind resistance is a major factor but that aside it would seem to me that 2000 vs 2300rpm would slurp some more gas. The 4.3s cost another ~$400. Right now 1-2mpg difference over the course of a year with $4+/gal gas is a few hundred difference. If I have the vehicle for a decade or more (ideally my plan) that could be $3000-4000 difference (or more if gas keeps climbing...). That cost is reasonable to bear IF I'm reasonably gaining something with the 4.3s. But in my use cases am I? Running <1500lb pop up truck campers, currently towing <7000lb travel trailer and fairly unlikely to tow more than 10000lb. With the low end torque of the 7.3 it doesn't readily seem like 4.3s are needed for that BUT I wish there was some notable testing on the two configurations in real life, maybe the 4.3s make a bit of difference in how much downshifting, dunno.
