RG-59 is fine, and assuming your outdoor antenna moves (raise/lower, rotate), its flexibility can be an advantage over RG-6. The difference at 900 MHz (high end of UHF TV) is 4.3 dB per 100 feet. So the difference for the short distance from the antenna to the jack in a typical RV is mice nuts. Same for the couple of feet from the wall plate to the TV.
Sure, you might see a difference with 50+ foot runs in a home, but for an average size RV, the smaller size and greater flexibility of RG-59 may give it an advantage, with little disadvantage.
For outdoor use, if you want to be sure it will last, try to find something made with name brand cable - Belden, Alpha, or Times Microwave.