Depends on lots of varibles such as transmission, gear ratio, weight of right foot. My Sub use to get 19ish but now gets about 17 with 700r4 trans, 4.10 gearing, 33" tires (just put on 35" so the jury is out on those). My Blazer will get 21 city/ 24 highway with 3.73 gearing 700 trans, 32" tires. 12 mpg seems low but I'm not sure what a "value van" is.
Mark Chapman DP member #653;
1983 K2500 6.2 Suburban, 4" lift, 35" tires, ATS turbo, Banks exhaust/intake, pyrometer, tachometer;
1986 K5 6.2 Blazer, 2" lift, 33" tires, Banks intake, pyrometer, tachometer
1963 wife, one owner, average mileage for the age but in excellent shape, a keeper
1992 daughter, low mileage, pretty, limited edition, but requires some money to maintain
1995 son, sports model, very fast & peppy, time will tell on durability and maintenance costs
"Grease is good"