Although I don't carry them myself because they get changed annually and get little mileage, most experienced club members would have extra primary and secondary fuel filters on board. Many also have a spare alternator, and of course a belt; I think the alternator, being commonly abused from poor dry camp practices, may be your best item to have a spare for, aside from fuel filters. And know ahead of time how to switch filters, so you don't have trouble with trapped air in the lines, etc.
Additionally, you might want some top-up fluids such as ELC or regular coolant, engine oil, a gallon of boiler fluid for the hydronic unit, a gallon of bug-off windshield washer fluid, and perhaps oil, coolant, and filters for your generator, because sometimes Cummins/Onan retailers are few and far between, depending on where you expect to travel. Some may have extra hydraulic fluid too.
Certainly, you could stock up with spare hardware and fluid for every component that uses them, but on my rig that would tax my storage space, so I think each of us has to make yea or nea decisions here according to our space and pocketbook, and perhaps how often we are in the boonies, say in the West or traveling to Alaska.
I'm sure others here carry all sorts of other backups, and will chime in. I've heard of fellows that carry air filters and tires, but those things eat up a ton of storage. A few things inside the coach have given me grief, so I've stocked up on the spindles that secure window shade strings at the bottom, halogen bulbs for the round ceiling spots, and cabinet catches for doors and drawers.
Joel
