Note pad and pencil to write down all the things you need.
On most rigs you are limited by storage space and weight. So a little creativity and thinking outside the box helps. Before just adding another item to your rig, think about any other uses for it, or can something else you already carry do the job. For example, I carry a rollup exercise mat for my daily workouts. When traveling, it fits excellent between our dining chairs preventing them from banging together. The rolls of paper towels wedge into the cupboard with the plates and cups, preventing them from moving around on the road. As much as possible everything needs more than one use.
It is easy to fall into the trap of carrying something 'just in case' or unneeded spares. At one time we were carrying two spare water filters and 300 feet of water hose. When I realized the most water hose I ever needed in the previous ten years was 50 ft, and changed water filters on the road only once, I pulled the extras from the rig. There was a small stepstool we never used -- removed it. And so on. Extra stuff just seems to accumulate over time. So ask yourself before even adding it to your inventory "How likely will I ever need this?"
Avoid duplicating tools you carry in your tow vehicle. Will the same lug wrench and jack work on both? The same small kit of wrenches and screwdrivers? Does your tow vehicle and TT use the same electrical fuses or taillight bulbs? Why carry two sets of spares?
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So many campsites, so little time...
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