I posted earlier in another thread (with the math), that to break even on the fuel costs with towing gas has to average $1.05 gallon cheaper than diesel. Otherwise the mpg difference between the two will quickly cover the delta. Especially considering some of these diesels get remarkable mpg already.
Wife's Armada gets 18.5 mpg on the highway. I get that putting around town and a hair over 22 mpg on the interstate. I was talking to the guy I bought my travel trailer from and he was pulling it with a 1500 Silverado. Best he ever saw towing was 10 mpg. I'm consistently seeing 13.5-14 mpg towing it and that's not on flat roads.
When it's time to delete this thing, I would expect to see upwards of 20mpg empty in town and probably closer to 25 mpg on the highway with my gearing. I've consistently seen at least a 3mpg increase after a delete. I would anticipate being in the 15-16.5 mpg range when towing the trailer that I have.