An air hose reel.

The unglamorous part of running on-board air, and the part that decides whether you use it. A loose air hose ends up kinked, driven over, and coiled wet into a corner where it degrades. A reel means it comes out straight, goes back tidily, and is ready next time.

It also solves the storage problem that makes people leave the hose behind — a 240 inch hose loose in a truck is a mess; on a reel it is a package.

If you have gone to the trouble of fitting on-board air, this is what makes it pleasant to live with.