Well don't do what I did. In my campervan conversion I lifted my Astro 2 inches, installed oversized tires, and put a rack on top to hold solar panels and carry a raft. (Picture below from yesterday while I was lifting the rear another inch)
I ended up lighter after the conversion than at start. My Astro was a passenger van and the rear bench seats, carpet, and pad are heavy. I pulled the interior, insulated with double-bubble foil insulation, and put in a plywood floor as a base. I don't have a toilet or grey water tank in mine. For the next campervan which will be for full-time RVing I going to use a Nature's Head composting toilet so I don't need a black water tank.
The biggest thing you can so is slow down. Even with the modifications for off-road use my Astro will still do 20 mpg on the highway at 65 mph. Increase that to 70 or 75 mph and the fuel economy plummets. I did a lot of research on expediter forums and there are people with your van getting 19 mpg fully loaded with 3 pallets running @ 65 mph.
Run your tire pressures on the high side - that is an easy 1-2 mpg. (I air down to 30 psi when I go offroad to reduce punctures and that drops my mileage on the return trip)
As to manuals - I'm a fan but not too sad that they are dying out. A reason for their demise you didn't mention is fuel economy. 5 to 6 speed manuals get lower fuel economy on the EPA tests than the 6 - 10 speed automatics.
