For my 4-8 trips from SLC-Moab per year along with 2-3 trips from SLC-St George, I will keep the V10. Truck/gooseneck has also done two trips from SLC-South Lake Tahoe. Same mpg, same experience, same feelings about the V10 vs a diesel. I sold the goose thinking I'd get the 10-11 mpg towing the single car. I just don't get that. This truck should just tow at 6.5-7mpg, I guess. If I was towing more often than that, I do think your argument for a diesel really makes a lot of sense. With how often I tow and how I use my truck.... I'll stick with it for now. As mentioned, I think I'd look HARD at the 7.3L gas before buying another diesel. These are my dollars. You may believe differently and I'm cool with that.
There are two hills that test the shit out of any truck with that kind of load on that highway (SLC-Moab----highway 6 through Price, UT). I'm at 62-65 mph going over the summit with that load.
I've bought rigs before based on other folks' opinions. I don't have them any longer. The above is my experience and my opinion. I'll stand by it.