(You didn’t realise were essential)
Jerry Can Fill this and then only use it when essential, as soon as it is used fill it at the next (cheapest) location to buy gasoline.
Multiplug This is a must, especially if there are 2/3 of you all with phones and cameras.
Shower Doesn’t seem essential, but decent showers on the road are rare, so you will appreciate the bag of water when you are stood naked amongst the trees having a wash for the first time in 5 days. Can be bought cheaply from Walmart or Canadian Tire
FM Transmitter or Universal Cassette Adapter Absolutely necessary for listening to podcasts and music throughout your drive.
Stove If you are traveling with friends a double burner would be a good investment, as well as a small stock of gas.
Fridge or Cool box Deciding if you would like a small fridge is a personal choice, dependent on if you would like some fresh veggies or cold beer. Start by looking on local classifieds, like craigslist or gumtree for inexpensive second hand choices.
Mattress For a typical car set up this must be something that can fold over when necessary, allowing you freedom to pick up hitchers or carry more people. This is the mattress I would advocate for it’s thin so easily folded, and extremely comfortable. If this is out of your price range keep an eye out for thick pieces of foam or mats on local classifieds. It is worth investing in something reasonably comfortable, if you sleep uncomfortably for months your body will start to feel it, especially after the amount of hiking and outdoor activities you will do.
Curtains Drill in screws and use curtain wire and hooks to be able to hang material for privacy whilst you sleep, if you have the skills and a sewing machine make your own.