I want to refer you to a professional, your personal use makes the situation much more complicated.
If the vehicles were business only:
Purchase of the vehicle is an asset, not an expense. Repairs are a deductible expense. Taking the vehicle on a road trip for purpose of the business would be a deductible expense using the standard mileage method (if it is a passenger vehicle under 6000 pounds) or using the actual expense method (for any other type of vehicle and for a passenger car if you prefer). When you dispose of the vehicle, that's the sale of the asset. If you sell for more than your cost, that's a taxable capital gain, if you sell for less, that's a deductible loss for the business.
Your mixed use makes the situation more complicated. I'm going to describe the general case as found in IRS publication 463, chapter 4. https://www.irs.gov/pub/irs-pdf/p463.pdf. This will limit your deductions as you will see. Whether there is a special provision for your type of use is beyond my expertise.
These would be considered personal vehicles that you also use in business. As such, you can only deduct the percentage of expenses equal to the percentage of business use. You will want to use the actual expense method because of the cost of your upgrades. You will need records of all your mileage—total miles driven, plus records of business miles that give the date, miles driven, and business purpose of the trip. You also need the total of personal miles, but you don't have to break down personal travel in more detail. You need records of all your vehicle expenses; this includes fuel, repairs, maintenance, insurance, and depreciation. Then you deduct the percentage of expenses equal to the percentage of business use. For example, if you drove 100 miles for vlogging and 900 miles for a personal camping trip, you could deduct 10% of your expenses as business expenses.
You will see these rules work comfortably for a traveling sales person, for example, but not for you. Suppose you install a $1000 engine mod, and the only reason you install it is for your video, you can still only deduct 10% of the cost if your business use is only 10% of the total usage. I don't know of any exceptions or alternative tax treatments, which is why you may want to consult a professional. To maximize the deductibility of your business expenses, you should only use business vehicles for the business.
