towing fuel milage

new to this forum, so go easy on me. i recently bought a 2000 7.3 powerstroke crew cab dually with 90k, got a great deal and was fairly happy with its mpg, roughly 14-16 everyday use no extra weight. recently took the fam on a camping trip pulling my 31ft 11000lbs bumper pull. my mpg dropped to 7-8 mpg what can i do to get better millage. it also seemed to be sluggish. could only make 35 mph pulling cabbage hill in the Oregon Blue mnts off I 84. big hill yes, but buddy with 2000 cummins could do it with ease no speed drop

ed