There are two issues:
V8 or diesel of some kind - V8 is thirsty but brilliant to drive and has a good reserve of power when towing.
Manual or Auto - We have one of each one of each on a V8, I much prefer five gears on the manual box to four on the auto, and for heavy towing the auto gets hot pretty quickly.
It's down to personal preference, but a good multipoint LPG system properly installed can give excellent figures on MPG, but you do have to curb the right foot a bit. 2+ tons of car takes a bit of power to push along, and with 3.5 tons of loaded trailer as well we are getting the equivalant of about 18 to the gallon on the level.
Peter