1. A variety of fuses in the sizes and values used by your coach, both house and vehicle. Don't forget the fuses on your converter. And make sure you know where all the fuse and breaker panels are, they aren't always obvious.
2.Duct Tape (Gorilla Tape is a good choice)
3. Spray Silicon Lubricant
4. Vinyl Electrical Tape
Most of the rest you will accumulate as you fix and modify things, you'll always have leftovers. Get a sturdy, multi-compartment parts container for them.
If you're using screws, bolts and nuts on the exterior or the bath/shower area, I suggest always using stainless steel, you'll be glad you did. I wish Winnebago did. If you have a skylight over your shower, I suggest replacing the screws with stainless. Some of my original screws in my 2002 Suncruiser were rusted to almost nothing.