We've had our Volvo for 6+ years. It gets an average of 9.6mpg towing, but I have no idea of mpg bobtail, as we seldom run bobtail. It is licensed as a coach/motorhome in MT with permanent tags, and no special driver's license is required. We only stay four places, so have a 6yo minivan for CO & MT and an old car in NC. We ride bicycles everywhere, but especially in Key West. Many hdters carry a small car or motorcycle(s) on the truck for sight-seeing/errands, but our lifestyle doesn't require that. If you roam the country, you may find an hdt less than optimal.
However, an hdt will handle any fifth wheel far safer than any other tow vehicle. We've had two incidents that could have been far worse had we not been in an hdt. The first incident two years ago was being rear-ended on I-70 west of St Louis by a big bus. Had we been in a pickup with a typical rv hitch, the 20,000# fifth wheel would probably have come though the cab. The second incident last week was hitting a big deer with the truck. The truck's mass threw the deer clear; therefore not damaging the fifth wheel, and only damaged the bumper and hood, with no other damage. I drove on with no problems. When maneuvering in cgs, the hdt will turn sharper than any pickup, and hitch placement well behind the rear wheels makes the fiver track much better. I can climb hills and stop better than any pickup, and the Jake brake makes long downhills safe and comfortable.
Again, an hdt is certainly not for most people, but those of us who have them, love them.