Hi! We have the Dodge 318. Loaded weight is 8700 pounds. We weren't quite that heavy. We were pleased with everything until we ran into a couple saying they were getting 18 - 20 on same rig but 3 years older! We purchased it used with 13K ... this was our first trip of any lengh and we did "enjoy."
I have a 1999 190 Versatile with the 318. I get 12-13 mpg around town and 14-16 on the highway. Have never got over 16. Have you replaced your engine air filter? I believe it's scheduled replacement is 15000 miles. I replaced mine at 12000. A dirty air filter drops mileage.
You could have your Dodge dealer check the computer for fault codes. It could be a bad sensor causing a rich air to fuel ratio.
If you're using the generator, remember that it uses gasoline too.
Watch what others tell you on the mpg. Some seem to want to up it for some reason. Good advice from smallcamper. Also check the air pressure in the tires and make sure the transmission is shifting properly. Fuel filter should alsso be changed.
I have a 1988 roadTrek 318 dodge. I get 18-16 on trips and 10-12 in the city. 170,000 miles on it. The last trip; from DC up through Maine across to Thunder Bay and down the Missippi to texas. looking to sale it now that I'm back home, $2500.00 but I busted the A/C so it is a fixer uper