My Onan is a 6500 watt version with 3,000 hours and I have only changed the cooling fluid for the fresh water side once (my bad). Having said that, the Onan runs like a top and starts instantly. I think I already said it in another message, the tech recommending it is not a specialist. Also, Last summer I replaced the heat exchanger which was leaking water at the end caps. Eric advised using an auto radiator cleaner which is essentially what the tech is suggesting. I'll probably do that or something similar.
I'll be flying back to Florida from Holland after New Years and look at the situation up close. Thanks for your help.
S/V Pacific Cool (SM 141)