What kind of gas truck is your F150? Model year, engine?
I've got a 2011 F150 Raptor, an 08 tuned and weight-reduced 6.4, and a 97 7.3.
The raptor, 6.2 gas, gets 13.5 mpg highway at 75 with 35s, tune for 93 octane from 5 Star, previously with a non premium fuel tune, it was about 10.5 mpg avg.
My 6.4 with a 6 inch lift and 37s, 14.5 mpg is the best it's ever got
My 7.3, tons of engine work, 6 inch lift, 37s, best it ever got was 17 mpg (stock engine, TS canned chip), now it's worse than 17
Diesel is no longer more expensive than premium gas here, but still it's less expensive overall to operate my F150 for daily driving. Fuel savings will never make up for the premium cost of a diesel and premium maintenance costs.
A diesel is just more fun to drive. They have more power, better drivability. You can achieve similar mileage of a stock gas truck but have a 6 inch lift and 37s on your diesel. Towing small things will make a difference with a diesel too. My side by side behind my raptor going to work, I get around 9.5 mpg, behind the 6.4, no change in normal mileage.
If you replace year model and trim F150 with the same year model and trim diesel, you won't save money. Now if you have a 2015 F150 with a huge payment and get rid of it for a $10,000 99 7.3, ya you will make out in the end in terms of money you spend to operated your truck. Be sure to compare apples to apples when making the decision.