Diesel is needed for towing more than 10,000 lbs. of trailer weight. Less than that and gas is fine. For a camper weighing less than 10,000 lbs. a diesel engine is not needed.
What you do need are low gears, 4.10 if available and 3.73 as second best otherwise. Don't go with a gas engine and 3.55 or taller gears as it will reduce towing capacity by 25% or more and with a camper it will make for slower acceleration getting onto highways or passing semi's going up grades. With a load there is no gain in MPG with higher gears anyway and with even a 6-speed auto transmission the truck has 2 overdrive gears.
The mistake you made was talking to a salesperson as that added $2,000 to the cost of your truck. Always walk onto a lot and ask for the fleet manager or the sales manager as they work on salary and no commission. No commission to pay means the dealer/sales manager has that amount of money to use when negotiating the price. Equally important if you are dealing directly with the sale manager there is not all the game playing where you are going through a made up scenario with the sales person.
Want a good price? Go the last day of the month, talk only to the sales or fleet manager, go late in the day, and go in rested and having eaten within the last couple of hours. You then have as close to a level playing field as possible.
Understand that if a dealer sells every vehicle at "dealer's invoice" he still makes a profit as the factory discounts the invoiced amount based on the volume of vehicles sold by the dealer during a sales period. The sale of a vehicle to you can put them closer to the next category and discount break and sale them money on every vehicle they have sold during that factory sales period.
I have a diesel pickup and I can say that I have more trouble finding diesel fuel (none of the 3 gas stations in my town sell diesel), I pay more for diesel than I do for premium gas, I pay 10 times as much for fuel filters which need to be replaced every 10k miles with a diesel, I pay for DEF in addition to fuel, the truck takes more expensive oil and requires twice as much in the crankcase, has two batteries to replace, has a more complicated and less reliable emissions control system with more sensor problems, and increases the weight of the truck by 800 lbs. which cuts into the payload it can carry. It also is more prone to problems with water in the fuel, with cold weather operation with fuel gelling, and with biodiesel fuels.
An injector overhaul will cost $5,000 or more than a new gas engine for a truck. A blown head gasket on a gas engine will result in a repair bill in the vicinity of $1200. Blown head gasket on a diesel truck and you will be lucky not to end up with an $8,000 repair bill.
Diesels are also terrible for short trips. If when not hauling the camper around you are making trips of less than 30 minutes the engine is never getting to its proper operating temperature and this will affect many aspects of the engine's performance and shorten the service intervals as well. No such worries with a gas engine.
When you do get a price agreed upon for the new truck this is the time to ask for items to be added at no cost or worst case at dealer cost for the parts and no charge for installation labor. This would include tow mirrors and running boards. With my current truck I got the dealer to add the tow mirrors, the chrome running boards, a bed liner, and an aluminum diamond plate tool box with a collective retail value of $2,000. I sold the tool box for $350 on Craigslist.