Really, how much you carry depends on how far or how long you intend on traveling. Also, it depends on where you're going. Crossing the country? Better bring an extra spare tire and shed full of tools. Going an hour up the road and there's a wal-mart or hardware store 10 minutes away, then not so much.
We keep a staple of things in the trailer and then add/subtract based on the above.
One thing not mentioned is a roll of Eterna-Bond tape. I once scraped the corner of my roof high point on a low-hanging limb while pulling to our campsite. Being able to fix that tear was the difference between a happy trip and nervous trip.
