20-30 mpg?
Modern midsize trucks can't even achieve close to that.
Even with a fuel injected LS and OD, expect 12-14 around town and 17-20 highway depending on your driving style/habit (and rear end gear!). Anything over 60mph on the highway and you start to lose mpg with these trucks (huge bricks).
The older Cummins with a manual can absolutely get mid 20's. My uncle has owned a 94 Ram reg cab longbed with a cummins for probably 15 years now. He hauled my 67 from NY to SC for me (rode with him) and hand calcs (GPS verified) showed 17 MPG with a 2500 lb trailer and 4K pound truck. He regularly gets low to mid 20's unloaded on the highway. I have read the newer ones are way up on power, but emissions (and the sheer size and weight of the newer trucks!) have killed economy.