I have done it both ways covering hundreds of thousands rv miles and
I will try to address your concerns of mechanical break downs , cost and ability to get the job done.
My last 2 new trucks were gassers to tow a (23 ft box)6000-6500# TT. Being that I do not need any special after market add-ons to tow my TT, I found it easier to lease my 1500 HD 5.3 gasser trucks with standard factory options so I can roll over into a new truck every 3 years with less than 36,000 miles. I'm always under my bumper to bumper warranty for repairs and honestly 'never worry about breaking down' although I realize new vehicles can. They are working !
When looking to buy a new truck I was searching for an advantage of a new diesel because I liked my diesel when I had one. I figured that I would ask the guys that 'use and repair' U.S and euro diesels.
My personal friends that have commercial hauling businesses are having diesel issues with new U.S. medium duty Ford 350-550 diesels and their down time is taking it's toll on them. They have a back-up truck , then another back-up for the back-up. GM and Ram motors are working ok but those trucks in the 550 medium duty apps do not hold up as well as Fords. That includes front ends and transmissions . A few euro brands and the GM 350 diesels with the Allison seems to be holding up the best for light duty hauling. If I needed a diesel to haul weight, that would be my choice today.
The added expense for a diesel in my towing application would an 'added expense initially' and then the 'added maintenance'.
The gasser in conjunction with the right factory gears and max tow package of any brand will do the job satisfactorily and what was advised to me. So far it has worked flawlessly, I could not be more happy. I felt the extra money involved would be put to better use for that rainy day for someone who is retired.