A diesel is great for towing 12,000 pound and heavier trailers. For camper use it is like having a Hummer to go grocery shopping. With a gas engine it does help greatly to have low 4.10 or 3.73 gears so the engine can spend more time in its power band without a lot of gear changes.
I would go with the new truck which is going to have air bags, ABS breaks, and often a 5-speed or 6-speed automatic transmission instead of a 4-speed. The extra speeds are overdrive gears.
It is also easier to find OEM and aftermarket parts for a newer truck, and that includes a situation where you need a water pump or alternator replaced when out on a camping trip.
I would be careful to note both the gears used (check using the VIN) and the capacity of the gas tank. Many older trucks have 26 gallon tanks while newer ones can have 36 gallon and the extra 10 gallons translates into more range before filling up again and when hauling a camper the MPG will be at its worst.
Diesel engines cost more to buy and twice as much to maintain and four times as much to repair. Easy enough to check the cost of a head gasket replacement or new set of fuel injectors for a diesel engine- it will be a shock but better to get it now than when you have a truck that needs to be repaired.
Diesels also require more frequent oil changes and fuel filter changes and need two batteries for starting. There are also far fewer places that can do diesel repairs which is again a factor when out on the road on trips.