When you enter an item to be depreciated for business purposes, one of the entries is the percent of business use based on time.
Keep track of the use of the toy hauler between personal (vacation), and each of your businesses. You can use any method to calculate the percentage allocation (hours of use, days of use, etc.), just be consistent and keep records of your calculations. Then, enter the appropriate business percentage as part of the Asset information for each business. TurboTax will question you about what type of asset you are depreciating -- trailers should be on the list for you to select.
You are correct that the toy hauler would not qualify for the standard mileage deduction because it is not a stand-alone vehicle. However, when standard mileage is used, the cents per mile is divided among various categories of vehicle expenses and a portion of that cents per mile is allocated to a depreciation equivalent for the vehicle, even though actual depreciation of the vehicle is not taken when the standard mileage is used.
@kenaslan
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