we sell American Eagle wheels for duallies in a 20x7.5" size for a direct fit on your 8x6.5" bolt pattern.
Dually - Eagle Alloy Wheels We have both the series 058 (holes) and series 056 (spokes) available. Complete set of 6 aluminum wheels, hubcentric, with all hardware to install. Give me a call. Dave @ Custom Performance Products, 330-503-3213.
These are really designed to mount 20" offroad type tires on lifted later model 4x4 duallies, but all the hub/bolt pattern specs are the same for the new Dodge trucks as the old motorhome chassis like yours, no reason they should not work. You would need to source an appropriate tire size for your project that will work on a 20x7.5 wheel.
If you are not after an aluminum wheel like that, you could move up to a 19.5" steel wheel like they used on the old Chevy P-series bread vans. You can usually locate them in junkyards. They are 19.5x6.5 with an 8x6.5" bolt pattern. You would need to have the center bore opened up to 4.77" for your Dodge chassis vs. the 4.56" center hole on the old Chevy trucks. Any local machine shop can do it, we use new accuride wheels like that when we sell the Eagle 19.5 aluminum wheels and need 19.5 steel inners to match. You can buy new accuride 19.5" wheels from Rickson Truck
Rickson Wheel Manufacturing already machined for your Dodge, talk to Dan over there he is very knowledgeable and can supply trim rings etc. as well.