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Hello and thanks for using Just Answers. A dump truck would be depreciated under MACRS with a 5 year life. The first year you would be able to depreciate 20% of the cost of the asset (assuming you are using the 1/2 year convention). You are correct that this is not an "auto" and therefore the listed property rules do not apply. Here is a link to the IRS depreciation guide. https://www.irs.gov/uac/About-Publication-946
In addition you'll want to think about taking bonus depreciation or IRC 179 to accelerate the deduction.
I should add that for tax purposes you don't get to use the useful life and instead have to follow the asset class provided in the IRS table (originally published in Rev Proc 87-56)