In general they're decent engines. (I think you'll find that the chassis is actually called an E-SuperDuty that year, but that's just a nomenclature thing.) I have a '98 Coachmen with that chassis and engine, and have no big complaints. It is a little noisy and unrefined, to be sure, but seems to have sufficient power for anything I've needed to do. The 4 speed transmission works, but is not as refined as the 5 speed that replaced it and does not have the useful tow/haul mode. The gap between the top two gears is the same (and rather larger than one may wish) on both transmissions.
Make certain that the spark plugs are properly torqued with a torque wrench whenever they are changed. As was mentioned, these engines do have a tendency to spit out plugs due to having insufficient threads in the head. It's possible to repair when it happens, but does not make for a very good or economical day, to say the least.
I seem to get somewhere in the range of 8 mpg or a little less on the highway. I would expect much the same for you. The aerodynamics of a class C are horrible.
