A diesel powered truck with a heavy camper makes for faster acceleration onto freeways and faster passing on steep upgrades and for better grade control on steep downgrades. For that you pay an extra $9000 for the trucks.
A gas powered truck with a large V-8 engine and 4.10 gears will perform nearly as well or at least well enough for what people actually need, not necessarily what people may want. There is an ego aspect to wanting the truck with the greatest payload or best tow rating or most powerful engine that is very seductive for many.
I bought a diesel pickup in part thinking that I would get greater range on a tankful of fuel. I was only partly right. The reality is that I need to be more careful to make it to the next station that happens to pump diesel than I did with less range and being able to fill up at any gas station.
If you need to tow a trailer that weighs more than 10,000 lbs. a diesel is very nice to have and if the trailer weighs more than 13,000 lbs. it becomes a need to have upgrade. Diesel engines are by no means needed for hauling a 6,000 lb. camper around the country.