I would go for the diesel. I had a gasser that towed my 5th wheel well enough in Florida. But I don't live in FL anymore and I traveled across the country while I lived in FL so I did encounter plenty of hills and mountains.
The maintenance isn't any more expensive than the gasser because of the longer service intervals. Fuel mileage is almost double from the gasser and 50% better when towing.
The pleasure of pulling with a diesel is obvious the first time you climb any type of hill or mountain; more so for the mountain. You will ask yourself 'When does the power/torque quit coming?' The answer may seem to be 'Never!'
As many posters have said if your wife is looking at fifth wheels and the ones she is looking at are getting bigger, get the diesel. You don't want too small of pulling power. A DRW vs. SRW will be better for pin weight and carrying capacity. I will be upgrading to a DRW from a SRW. How soon depends on how my truck handles the new toy hauler.