I have 1500 hours on my Onan and never been able to get close to 200 hours from an impeller.
The best remedy I could come up with was to buy another pump and interchange pumps with a new impeller installed, making an iron rule to never run them past 150 hours. I have had no problems since.
I do have a Speed seal installed on my Yanmar raw water pump I have found it to be an excellent product which has dramatically reduced wear on engine impellers.
Spare end caps and seals for the heat exchanger are invaluable.
