If you collide with another vehicle, with a tree or a building or a big rock, or roll your truck, or any other similar event that causes damage to your vehicle, collision coverage is what will pay you for the damages.  Given the significant monetary investment that you have in your rig, collision insurance, while not legally required, is in the way of common sense considered mandatory. Note that if you are paying off a loan on your rig, the lender will almost certainly require that you have collision coverage.

For best coverage, the limits on your collision and comprehensive policies should be sufficient to replace your rig (replacement value) as opposed to merely compensating your for the current value of the rig (actual value).  This for the obvious reason that if you have an older rig that is destroyed or stolen, coverage for actual value might leave you with having to come up with substantial funds to buy another rig. Replacement value coverage costs more, but actual value coverage can be risky, especially if your rig is older.  As to deductibles, as always, the higher the deductible the lower the premium. In general, modest, not too small and not too big, deductibles are recommended.