SB truck for an AF865

When I spoke with a dealer about the 865 vs the 990 I was told it really doesn't make that much differenec. Weight wise you will not be a lot lighter (they list ~365lbs lighter than the 990) though perhaps they have the smaller tanks which would keep you from loading it up as much I suppose.

If you want to stay under GVWR than you need to look at a SB Dually. Otherwise, SRW trucks aren't rated high enough to carry these campers and stay under the limit.

Personally, I've ignored that, added air bags (a lot of duallys do to), stable loads and 4080lbs tires and said "to heck with it"

Ditto on the mega cab if you're worried about the weight, but you'll have to pack awful heavy to get to 4500 lbs. It will be close to 2 tons though depending on how you pack but I think the weights I've seen in here don't break 4k.
Any srw HD short bed pickup will handle the camper fine, but you won't be within the mfgs typically conservative ratings and you may want need to do some minor suspension mods.
Do not worry about hauling that on a new F350 of any configuration.

Personal preference, but give the built in generator some hard thought. They're convienent on one hand, but loud (inside), expensive, relatively inaccessible and use a lot of propane.
I mounted a Honda 2000 in the same "wing" area on our AF 860. Of course you have to remove it to use it, but you're not sitting on top of a running generator. JMO on the built in generator thing.

You can get a shorted mega cab Ram with over 5000 pound payload.