I would find some 915 or even 516 heads and get some tight quench and more compression in it. If you go 516 get bigger ex valves put in with a tiny bit of deshrouding in the chamber and round out the transisions in the exhaust ports. Like has already been mentioned keep the stock cam for very heavy loads like that thing will see. When dunnuck racing built a towing 440 they found big gains adding quench and compression, I am positive from my personal testing that would help with MPG also. If you need a new cam than get a custom cam with about 200@.050 and as much lift as possible on a 108 LSA. No off the shelf cam is going to give you much gain over what you have. That gear is not much with a very heavy load and very tall tires typical of a rig like that so you need help at very low RPM you can get.