I would say that the dually would only be a few hundred more than a SRW. I guess you have your heart set on new and a manual transmission? The automatics are very popular, and actually get better mileage on many cars.
Even engine braking is much better these days with a automatic than in years past. Also you are limited to towing less weight with the manual transmission, due to the fact that the truck and trailer must reach a certain speed while going up a grade and starting from parked, in order to meet the 'recommended towing weight', and still not burn up the clutch due to to much slippage. . .
So I would also recommend a auto transmission.
As for towing fifth wheels or delivering trailers for a living - not with my $50,000 truck, or a new one, especially. THink of it this way. The $50,000 truck will become a $15,000 truck in 100,000 miles or so. That is $35,000 hit on the truck's value by driving it the first 100,000 miles, or around 30 cents per mile. It is no wonder the RV dealerships can not find anyone to deliver their RV's for less than $1 per mile or so. And you would normally be making the return trip for free.
So say you need to travel 500 miles to Indiana to pick up a travel trailer, take it to say Florida, and drop it. Return home for free. That is say a 2,400 - 3,000 mile trip, and they will pay you for say 1,200 - 1,500 of that.
I understand that you would be able to tow your own gooseneck or fifth wheel delivery trailer to move campers, pop-ups, and such, and get paid to move 2 or 3 RV's at once, this would pay pretty good! But I don't think they will be sending a whole bunch of campers and pop-ups instead of travel trailers or fifth wheels.
I guess it would be impossible to predict that a truck going from say the Lance factory to the Lance dealership is hauling a camper for a fee or a personal camper on their own pickup. However I have seen some delivery trailers that have 2-3 campers on the trailer, so I know those are delivery of new or used units, not owner operated RV's.
So IF you happened to live near the Lance factory, and could haul one camper, and tow a 26' lightweight camper at the same time, then the dually will work.
You might also consider a used Dodge dually - you might find one with a 5 speed. Ford rarely installed 5 speeds, and changing to 6 speed manual did not sell much better, so they dropped manual transmissions on trucks less than a F-650. You likely will find it difficult to locate a used manual transmission.. . .
I don't know when Ford dropped it's manual transmissions, I know that the 6.7L Diesel did not have them, and could not with a straight face recommend a 6L or 6.4L Ford diesel. . .
Good luck,
Fred.