That sounds about normal.

As far as increasing it, well, that is pretty much the same as any other vehicle. Gentle accelerations, lower cruising speeds, synthetic oil, higher tire pressure, better engine breathing (in and out), etc. might net you a few mpg. Nothing miraculous though, the Trooper is a heavy, sturdily-built, high profile vehicle, which does not translate into high mpg operation.

Steve Carlson - 95 Trooper LS expo rig
Serenity now!