We have been full time for year, and i can say most of my spare stuff like our spare slide motor is still in the garage. I have had 4 slide cables snap, and thats even with checking them monthly and adjusting. They dont take up much room so i keep them handy.
I replaced our water pump with a "quiet" one and kept the original as a spare. We use it for a transfer pump now when boondocking.
Other than that the stuff that broke was major stuff like a front jack. But the small stuff saves ya everyday headaches, like spare hose washers and fuses.