I've seen some real creativity when it comes to doing this. In both cases it was a 67-72 Chevy fleetside trucks that had been converted to under-bed rear (Suburban) tanks.
One of them had one of these tucked into the rear corner of the box where the stake-pocket hole used to be. They had boxed in the corner with sheet metal and painted it the same vehicle color. I was impressed by the fact that it was readily accessible even if you had a load on.
The other truck had a concealed filler neck under a swing-away factory marker light. I'm not exactly sure what he had for a fuel cap, or how difficult it was to actually get fuel into it ... but I had to give him credit for maintaining a totally stock appearance.
My plan is to graft in an actual fuel filler door (a la 88 Chevy PU) behind the rear wheel, and build an enclosure that sits on top of the box-boards in the rear.