Ahh the "all knowing" sales guy. He's either really misinformed or as others mentioned, just trying to get you into the diesel, which based on your posted intended usage, sounds like you wouldn't need. Don't let them sway you, get what you want but I can confirm the 350 long bed/crew cab is not underpowered. There's some videos in this forum of mike (5 star) pulling his 5er as well as me pulling a 15k trailer wide open up a pretty steep/long hill.

My mpg's do suck (10mpg city/13hwy) but the ride is great. I didn't buy it expecting much better than my old v10 though...