Some of the newer designs of light duty full size trucks with V6's are rated at 17/23 or 16/22 or thereabouts. Real world you're probably talking 14-15mpg around town with mowers in the back.
The problem with smaller trucks is you either have minimal bed space or you have to spend a good bit to modify that. That adds weight and drag which might negate some mpg advantages. And I've personally found that a smaller v6 or I4 engine working harder doesn't do that much better when loaded down than a 2wd small v8/good v6 full sizer working less hard. Compacts are also built really light and fragile.
V6's vary though. it used to be they put old pig engines in because they were cheap.
A 2wd long bed reg cab is lighter can will give adequate power vs a 4x4 crew cab towing a trailer, so you'll probably find a full size truck more versatile overall. The compact offerings are either overpriced (toyota) or too small to hold much w/o a lot of modifications.