Diesel is the ONLY way to go. Whether you go Short Bed, Long Bed, Mega Cab, Standard Cab, etc. As far as diesel there is less maintenance, but it does cost more, 3 gallons of oil. Yes Diesel cost more per gallon, but if you drive it daily you will make it up quickly, 17MPG around town, 22-24mpg on highway and about 12mpg towing, OK maybe not your trailer because it would not even know it's there.
Go diesel now that way when you Upgrade your trailer you don't have to Upgrade your truck AGAIN.
I don't have one now but I did have a 2003 Cummins Dodge Ram 3500 QC 4X4 SB towing a 12K 5th wheel. I do miss it but she would out tow and out pull most everything out there and get 17MPG city and 12MPG towing
Retired Navy Submariner
2014 Itasca Sunstar 35F; 5 Star tuned; 2014 Jeep Cherokee TrailHawk