Cummins conversions take a long time to pay off, if ever. While I love seeing them done, I though long and hard about doing it to my stepside, it depends on the trucks usage. If you're going to use it like Zimmer, then its a good fit. If you can find a screaming good deal one that helps also.
My truck will always be a secondary truck for me that I simply enjoy having. So sticking with the 6.5td is the best choice for mine, cheaper too. Plus I have my dodge which can pull more the my stepside can ever dream of safely pulling.
As for mileage, my dodge averages around 17.5 overall, mix of highway and city. When I was pulling my friends jeep/buggy which we figured was around 4500lbs on a 2000lbs trailer plus several hundred pounds of tools, parts, camping stuff, ect. Would get about 13 with the cruise set to 75 mph, 11 running hard at 80/85. Truck itself weights 8000lbs with just me and a full load of fuel. This is going from the jersey shore up to Rausch creek, so running through hills most of the trip.
My truck isn't stock either though. I have Banks six-gun, usually on level 4 of 7 even when towing, 50hp injectors, Arson 2 kit on the cp3, Banks big-head waste-gate actuator. Plus a full trans from Goerend with billet input shaft and flex-plate. I run 305/65r17 procomp xtreme all-terrains. So she'll definitely haulass even with a trailer. I never did much towing with it when all stock nor have I ever towed with a 12 valver but that's how mine runs.
A picture to see what I'm running

. Mine the silver one. The green one is a another friend, its an 02 24 valve that is at least on par power wise to mine if not even a little more.
