Towing 20,000 + lbs. the diesel will run around a gas every time. Just the torque difference is what makes the difference when pulling heavy weight.
I'm going to attach a weigh slip. It's one of those individual weigh slips (the kitchen side of a toyhauler will always weigh more than the other side). My triple axle Grand Design Momentum 376TH is 42' with 5 slide outs plus my bike in the back. Loaded, just the trailer came in at 21,025 lbs. so in the ball park of what you're talking about towing.
I pull it with a 2014 Ford F-350 Crewcab, Longbed, FX-4, Dually. On this weight slip you'll see that even though each individual wheel and hitch pin weight were all under the maximum, the overall GVW on my truck was over by 125 lbs. and the overall GVW on my toyhauler was over by 1,025 lbs. and that's with a dually! I've since shed some weight but I have everything I own in here. We are full time RVers and have been ocean to ocean in the past year.
Mileage is what it is when you're pulling this kind of weight....but it isn't bad when we're running empty. Rode up to Yosemite National Park yesterday and as we came out of the north entrance, I noticed diesel fuel was $4.89 Gal. so I refer to my Gas Buddy app often. I try to fill up when I'm unhooked because diesel is much cheaper at a regular gas station than it is at truck stops.
I will suggest that with Ford, do not get one older than the 2013......you want the 6.7L which has proven to be a real work horse . This motor was designed in house by Ford and the tranny was designed to go along with this motor. I just cleared 90,000 on this 2014 that I bought used and no issues. Regular oil changes every 7,500 miles and I use full synthetic since I'm pulling weight so often. The $$ is determined by the level of comfort and bells and whistles. You can get an XL or XLT for less than you can get the Lariat pkg. that I have and my Lariat is less than the King Ranch. It's all just a matter of choice if you're trying to stick within a budget.....newer truck for the same price will be a lower model without all the bells and whistles. How somebody takes care of a truck, service records, etc. determines a lot for me.....so I bought from a used truck from a private party that had all the service records.
By the way, most RV Mobile Techs are also qualified to pull fluid samples (oil, tranny and coolant) in any vehicle. I even did the oil in my bike at 50k miles.
I'm not an RV Mobile Tech. however, I did go to RV Maint. Tech school in Texas last winter and I stayed on for the training to become certified as a Level 1 and Level 2 RV Inspector certified through NRVIA. Took all of that training more to know how to work on my own RV. Sure don't claim to know it all but if you're looking at a unit and have any questions, just shoot me a PM. I would suggest getting it inspected by an RV Inspector (one that's certified, not a shade tree RV mechanic) prior to doing the paper work. It's money well spent. They'll spend 6-8 hours going over, under, around and through that RV and then a couple of more hours generating a report with photos, etc. and they will complete fluid sample analysis on the generator oil, also....if you'd like (as well as any truck you're looking at buying).
Fluid analysis is the next best thing to tearing a motor all the way down to see what's going on with it.
Hope I didn't overwhelm you with all of this. Just trying to help.