I carry 2 x5 gallon tall, thin jerry cans strapped securely behind the rear wheel wells under a locked truck cap with no windows.Have used them a couple of times in the new truck because of small OEM tank.
And contrary to what some have said on this thread, I have run into situations in the western states and northern BC/Yukon where gas stations are either far apart ,are closed early because of winter hours or the odd time have run out of diesel.
Dave