Ford E450 V-10. Problems with #10 cylinder

I have a 1998 Four Winds with a V-10 and 39K miles on it now. Bought it last year and had $3200 of preventive maintenance done on it. The only thing I had done to the V-10 was had the engine and radiator flushed and mass air flow sensor cleaned. Had all the hoses replaced, new shocks, tie rod ends ect.... Just came back from a 2400 mile trip. The V-10 was perfect, mpg over all was 13. You are not going to speed anywhere in Oregon giving great gas mileage. The garage that did the work said they have seen blown spark plugs. One time on a V-10 but more on the V-8's. I love the power.