No matter the diesel engine, mileage is dependant on gearing. The early 5.9L Dodge trucks had 3.07:1 gears and broke 20MPH, the uprated engine went to 3:54-1 with OD and stayed above 20. But 4:10 gears designed for towing mathematically 20 is hard to get. I think if, and I mean if I ran 45MPH, for a full tank, sure 20 is going to happend, but not at acceptable speeds.
My old Cummins Chevy is now a farm truck. Would NOT think twice about a cross country trip, even if its a 93 model, but for mileage, I drive something else.
Walter, if the 6.5 starts well,has good oil pressure, replace the chain or install the gears and with just over 200K, another 100 is possible if not more. If you are not pulling 12K or more, it will do ok. The injectors on the 6.5 are very forgiving and a bottle of cleaner is tune up enough. Hydraulic llfter, and most everything adjust similar to a 350. After 94, the injector pumps and PDM cooling issues cause problems if pulled heavy. Used as a pickup for errands, normally fine (must be with 200K).
Sounds like you got it at a reasonable price. I wish you well.