Here in Central PA, a number of the large natural gas companies, including Haliburton , have replaced their light duty diesel trucks with newer gas powered trucks. Halliburton is now using gas powered F350, extended - crew cab trucks with long beds, loaded with bumper auxiliary fuel tanks, and tool boxes. I know Halliburton's reasoning was decreased maintenance cost and the fact that for their use in this area, the cost benefit of diesel vs gas was negligible. None of the employees I've talked to complain about power or comfort, and they are living out of their trucks (sometimes literally) for two week stretches at a time.
OP, sounds to me that you could go either way. A 3500 lb trailer isn't much and almost all modern trucks can tow it without issues. A gas engine would work fine probably. With a diesel, you might find you have more towing capacity and you can get a larger camper or do more things in the future.
Lastly east of the Mississippi anyway, there is very little difference in the price of a few year old diesel with less than 80k miles. In that age and mileage range, there isn't much depreciation on diesel powered trucks. You could buy a diesel if you want and sell it private party for close to what you paid for it a year later and replace it with a gas engine model.