I have an F150 5.4L as well and it pulls nice. My travel trailer is about 7000lbs loaded and pull and stops with no issues. I bought the Ford mainly due to the price and inside capacity (I have two boys). That said, if the decision is about a chev 6.0L and a duromax...the diesel is $7200 more.
If you say you will get 5 MPG better while towing with a diesel than the gas. At $0.80 per liter you would have to drive 197,000 km just to break even. I guess the real question is...how long will you own the truck? and is there more reason that just fuel consumption?
For the math I used 15MPG for the Gasser, and 20 MPG for the diesel.
Math:
15MPG = 5.33 km/L
At $0.80/L it costs 14.9cents per KM
20MPG = 7.11 km/L
At $0.80 it costs 11.25 cents per KM
14.9c per KM - 11.25c per KM = a difference of 3.65c/km
$7200/$0.0365 = 197,260KM
but does anyone actually get 20mpg towing with a new diesel? or gas. i went to Nelson bc just after i got my truck. pulling a 4 place sled trailer at 120kph average and i got around 16l per 100k or about 14 mpg as per the gauge in the dash. diesles now adays i would suspect the same. so would take even longer to pay for itself. unless you really need the towing torque and payload go with the gas.