1993 motorhome--454 engin--12000 miles engin seems to hit fine but have lost about 1/2 power--"service engin" light on--as I recall there is a way to get trouble code that might help---turn key off and on 3 times or what?? help please

Somewhere under the dash near the driver side should be what's called an ALDL (for Assembly Line Diagnostic Link), it's a black plastic connector with two rows of 6 pins. It might have a plastic cover on it that says "DIAGNOSTIC". If so, remove the cover and underneath, you will see the connecting pins (they're not really pins, more like little sockets). On the upper row, the pins are lettered A-F, with A on the far right side and counting leftward. Shunt pins A and B with a paper clip or small piece of wire, then turn the ignition On but DO NOT START THE ENGINE. The malfunction indicator lamp (MIL) will, in groups of 3, flash the diagnostic trouble codes (DTCs) beginning with a Code 12 (not a trouble code, more to let you see the system is operational). It will flash a Code 12 in this manner: flash-pause-flash-flash. It will do this 3 times, then it will start flashing actual DTCs in numeric order, always in groups of 3. For example, a Code 45 would be flash-flash-flash-flash-pause-flash-flash-flash-flash-flash.