My RV Onan Genset 4000 generator will no longer crank, just clicks.?

The unit is circa 1993 and has only 75 hours. I've had it for 4 years and no probs. Last week went to start and only got clicks, like low battery or poor connection. Battery is new but tried other even newer batt just in case. connections look clean. tried pulling spark plug but still would not spin, just click, click, etc.