Two things

Sites and forums dedicated to living in vehicles are filled with lists of essentials. There are even videos showing all the items you need and how to use them. But if you're living in a vehicle so you can travel, then there are two items that rarely get mentioned.

If you're going to be wandering the United States, visiting a lot of National Parks and Monuments, and camping in Forest Service or Bureau of Land Management campgrounds, then you'll probably want an Interagency Pass.

If you're a young fart, an annual pass will cost you $80 a year. If you're an old fart (62+), the senior pass is $10 and good for the rest of your life—a life that will be enriched by travel and being in nature.

Whoa, $80 a year for a pass? Well, let's say you want to do the trifecta of Grand Canyon, Zion and Bryce Canyon while you're in that region. Grand Canyon is $25, Zion is $25 and Bryce Canyon is $30. That's $80 right there. And using a pass will get you through the gate quicker than if you had to pay each time.

If you're a senior, your lifetime pass is also good for 50% off the price of federal campsites.

No states that I know of accept the federal Interagency pass. They want their own sources of revenue. So check to see if a state you plan on visiting a lot offers their own annual pass.

The other item is a passport or passport card. (Or and Enhanced Drivers License, currently issued only by New York, Vermont, Michigan and Washington.)
I would hope everyone already has a passport, that everyone has had the time, resources and desire for foreign travel. But I know that's not the case.

Sure, it's a hassle getting a passport, and you might never need one. But someday down the road you might want to drive to Alaska, which means going through Canada. Or you might want to go to Mexico for low-cost dental, optical or medications, or for warmth in the winter. You'll need a passport. Ever since 9/11 US citizens have needed a passport to get back into the country. C'est la vie. Así es la vida.

If you already have a passport, check the expiration date. Mine expires this summer, so it's time to start the renewal process. If you don't have a passport and you haven't hit the road yet, it's easier to get one while you still have a fixed address. Do it now. Here's where to get started. Then maybe we'll meet one day on a beach in Baja.