I was in the same boat as you. Just bought a 5th wheel that is 14k gross. I wanted a gas for up front cost, maintenance, and the fact that I know zero about diesel.
The 5th wheel we were looking at was 10k gross and I would have done gas for that. Our previous travel trailer was 7200 gross and I always felt the truck could use a little more power on hills and accelerating at times. It was a 5.3.
We decided that since we save money going with a used trailer to get the diesel. I am glad I did, no stress when pulling. I had a friend pull his 5th wheel that is the same size with a gas and was surprised at how good it did. I think if your doing short trips and not a lot of hills a gas would do it, but you might always wish for more.