You won't have a OBD2 Port, yours will probably be a J1708 used in that Vintage Diesels.
See Picture for the physical configuration of the mating connector.
The best option for a trip meter is the Scangauge "D"
ScanGaugeD - Advanced J1939 & J1708 compatible Vehicle Monitor for Diesel Pusher Vehicles : Linear Logic
You'll get both a trip comp and a bunch of gauges four at a time.
Look for your diag connector somewhere around the steering wheel column.
Mine didn't have one under the dash but I found one on the engine and ran wires from it to the dashboard.