You should know that diesels generally cost more to maintain. Example-the Ford 6.0 takes 15 quarts of motor oil and a more expensive oil filter. Also-diesel prices have not been kind over the past 9 years as compared to gas prices. Parts are also more expensive because they are heavier duty. In Illinois the cost for plates, inspections, extra winter additives to prevent fuel gelling add up fast. But personally I am very happy with my 1 ton dually because that is what I really need for my rig.
I say this because you mentioned fuel economy but I think you should instead be looking at overall operating cost. Your trailer isn't terribly heavy so many 3/4s should do a fine job with either a diesel or gas engines. I haul an 8,000 lb utility trailer, truck and trailer fully loaded, with a ¾ with no problem. So buy all that you need but if you are also looking at cost per mile be aware that costs can escalate.