I think it is dependent on what is the "primary" purpose of the trip. If the "primary" purpose is for rental activities, it is deductible. If the "primary" purpose is for personal, it is not deductible at all (except for expenses relating DIRECTLY to the rental).
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"Primary" is usually based on the amount of time. So if the OP spends 4 hours per trip on rental activities and 20 hours on personal activities, the trip would not be deductible at all.
