2001 Silverado K3500 dually Duramax/Alison carrying '98 Bigfoot 3000 series truck camper. Camper gear and fuel about 4500 lbs loaded, only 5 gallons of water.
I use Optilube XPD fuel lubricant.
Fuel economy goes down about 1 mpg per 1000 lbs.
Best unloaded mileage is 19 mpg at 55 mph, loaded 15.5 mpg
Driving faster 65-70 it drops to 17.5 unloaded, 12.5-13 loaded
While i bought mine new, I would not hesitate to buy a higher mileage duramax.
Some issues. Add a 2 micron pre-filter for the fuel. Otherwise they typically need new injectors because the stock filter allow 6 micron particles would fit in the gap between the piston and cylinders of the injectors, causing them to wear out. Cost for new injectors is about $2200. If you can find one with replaced injectors, grab it. Look up Nictane Filter Kit.
Stock "drivers side" exhaust manifold has a restriction. It was made to provide clearance for the steering column. Chevrolet later decided to add the Alison transmission and put on a body lift. So the notch was no longer needed. They did not fix the exhaust manifold. So many people put on an unrestricted manifold which helps efficiency a small percentage and provides an EGT port.
Modern diesel fuel lost lubricity with the sulfur removed. Adding a lubricant like Optilube, biodiesel, or 2 cycle oil (ashless), make a big difference on diesel performance--about 10-15%.
To improve performance under heavy loads, a larger exhaust, or less restrictive air cleaner--which lets in more dirt. I choose not to go this route, but these do yield improvements at faster speeds and while towing heavy loads. I may rethink this as I plan to tow a boat in the future.
Using a boost gauge helps mileage. Low numbers are more efficient. I try to run at 1600 rpm with boost numbers 1-2, and up to 6 or more on hills. It helps to back off on the throttle.
Final thoughts: I have no problem parking anywhere. My only stealthy qualities are low level lighting when parked. I have no slide outs, R12 insulation, and have camped out in 9 deg weather. I worry a bit about water freezing, but have not had any problems yet. No one ever bothers me anywhere in cold weather. I burn a tank of propane in a 1 to 1.5 days in these temps. A little prep--improving insulation, and perhaps a trailer with a big propane tank and I could easily camp for extended periods in extremely cold temperatures. Try that in a van.
