Sounds like there are some heavy right feet around
I've owned four RRC V8 autos (one 3.9 & three 3.5efi) all bog standard & always got decent consumption, for a 2tonne perm. 4x4 brick. My current '87 has returned 22+ on a 600 mile round trip to Wales from Essex & I wasn't driving Miss Daisy, cruising at 70 on the M4 for much of the journey. Local rural driving averages 17/18 mpg.
nb. these figures are full tank to full tank calculations, not guesswork from fuel gauge indications.