You don't need diesel for a 12k lb fifth wheel but you'd like it, especially if you plan on towing out west. You also don't need DRW for that size fifth wheel, so unless you're planning on going dramatically bigger in the future I'd stick to your preferred variety of SRW - easier to park, better daily driver, better off-road or on tight forest service roads, better in snow/ice, and they look nicer (that last one is subjective and not terribly important).
I have a short bed and like it better overall since most of the time I'm not towing and the I prefer the packaging and look of the short bed, but I admit on some trips it would be handy to have that extra bed space.