For receipts and the such, why not just take a picture of it with your iPhone or Android? Doesn't cost you a penny then. Also won't fade like receipts do.
For mileage tracking check out the top 5 at https://www.gofar.co/blog/mileage-tracker-iphone-apps/ and consider the one that best meets your specific needs.
Overall, unless you have aggregated your multiple rental properties into a single activity, you'll find that having to track mileage for each separate activity is going to be a real pain. But it's nothing compared to the pain you'll deal with when you do something like sell only one of the properties, or if you sell or trade in the vehicle for another one. (Just a head's up on that, as there's no tracking app in existence that can alleviate that particular pain.)
