The answer to this is easy...though not a full-sized pickup. The answer, however, is 2wd Chevy Astro Van. You can get up to 20 mpg in the AWD version if you keep it near 70 and don't lift it. The 2wd should easily get you 20 mpg. Probably better if you keep highway tires on it, and drive conservatively. 2wd can be found by the bucketload in your price range. Stealth camping in the back is easy. Probably smaller than your current van but if my GF, myself and 3 full size dogs can pull it off, in winter, you can manage a surf trip easy. Bonus, most already have a rack on top for your boards.
By the way, as you probably know. Stealth camping is much easier to pull off when you can exit the driver or passenger door as opposed to climbing out of the tailgate of a pickup. You can also fit boards inside an Astro easily too. Plus many have rear heat and A/C which is awful handy for cold/hot weather and quickly drying out gear.
I'm serious. An Astro will suit you very well. Plus for days you want to pull major highway miles, they have no problem running at 85 mph all day long and can pull steep, long grades without much trouble. Look at 2003-2005 models for the best build...namely 4 wheel discs, and multiport fuel injection. The 4.3, as others have mentioned, is a pretty damn good motor. Mine is at 216k with VERY little maintenance and it runs like a top. There's a reason contractors love these vans.