How disappointed will I be

2 years ago I swithced from a 6L gasser to a new D-max. The only difference between my 09 and my 13 was the motor/tranny. I went from an average of 6mpg to 10mpg towing. We tow a 31' TH with one ATV and three dirt bikes. Although I don't drive the TV much for other than towing, the mileage with the deisel is always better. As another poster mentioned, the volume of noise in the cab is much greater with the big gas motor. Either way your fill ups are going to be $100. I've heard the new Ram 6.4 gets 10+ towing loads of up to 12k, but I have a hard time believing it.

I agree your best bet is to go buy a commuter car.