The only reason why I considered a diesel were as follows:
1. Wanted a vehicle that was turbocharged, and there's no such thing as a turbocharged gas HD truck as of this posting. I live near a bunch of mountains and my 2010 F-150 with gear and my wife was out of breath in Wyoming. I didn't want to think of it with an extra 8k or more behind it.
2. The wife's taste in RV's are quite.... large.... So I figure I'd be in the 10 to 12k range for an RV. So my half ton wasn't going to cut it for payload capacity and reason #1 above.
3. I wouldn't mind owning a turbo-gas vehicle, but the only HD turbo vehicle made today is a diesel. However in my case, diesel tends to only be around 10 to 15% more expensive than regular gas in my parts. So I still come out ahead on fuel costs when cruising in my part of the country, especially with the miles I plan on throwing down in 5 years. Currently, I don't care so much about diesel mark-up since it pays for itself at re-sale and the maintenance is actually not that much more expensive than a gasser if you're a DIY type of individual.
If items 1 & 2 & 3 don't apply to you (and it sure doesn't seem like it), I think you'd be perfectly fine with a gasser. I remember my brother hauling a decent sized camper (roughly 6k) with an F-250 that had the ol' 460 V8 in it. Sure it slowed down in the mountains a bit, but it still did the job just fine. Being in the flatlands, I think the only issue you'd ever truly have is high winds.