Karen, if you were looking to buy new, I would recommend a gas engine truck for personal, average mileage use.
Slide-in truck campers don't stress a good sized gas engine like towing an 8000 pound travel trailer does.
And the new trucks with large displacement V-8 gas engines have plenty of power, and the cost-per-mile overall will be less than diesel. Sometimes a LOT less. Gasoline fuel is available just about everywhere that people can be found, and is often a lot cheaper, so the MPG cost usually balances out.
Brand new diesel engines are crammed with emission control systems and devices..and those are known to reduce power, economy, and reliability...in general. Out-of-warranty repairs can be scary expensive.
Now, having said all that about NEW engines, if you are looking for used, consider both. Keep an open mind.
If looking at used, the options are many, although finding a nice low mileage diesel at a reasonable price is a bit harder, and often the history of the truck with very high mileage on the clock is unknown. Plus, some major problems can be hidden...of course that's also true with a gas engine but they don't cost as much to replace if it ever came to that.
Often you can buy a nice 4WD gas engine truck for the same price (or less) as an equivalent 2WD diesel engine truck. And most earlier model 3/4 and 1 ton diesel pickups do NOT have engine or exhaust braking....a real detriment to safety when towing or hauling heavy campers in the mountains.
If you were looking to pull a large travel trailer, all over the country, I'd surely suggest a diesel (with auxiliary braking). But a slide-in truck camper is not a problem for a newer gas engine, as long as the truck chassis is 3/4 or 1 ton rated.
