Last year, we added a USB Combo outlet to the interior of the RV, and we found that it was so useful that we decided to add one to the exterior, by replacing the existing 120VAC outlet on the side of the coach. Whether or not you can do this on your RV depends on a few things.
The chief difference between these combo outlets is the available power of the USB charger. The best outlets are capable of providing 3Amp or more, while I have seen some that supply less than 1Amp. Personally, I would go with one that supplies at least 3Amps, however the higher the current capabilty, the larger the unit tends to be.
But avoid those units that supply less than 2Amps as they will take too long to charge a USB device.
First, the USB outlets tend to be a bit deep, typically at least 2" in depth. This means you will most likely have to change out your electrical box for one that is deeper. This may either be a very simple or extremely difficult task.
Assuming you can easily change out the outlet, you will need to find a weatherproof cover that is intended for GFCI outlets, as those have the rectangle opening that is required for the USB combo outlets.
I was not happy with the way the waterproof cover fit, especially the gasket. So I simply silicone caluked the outlet cover in place. These covers are not expensive, a couple of bucks for many of them, so it is not a huge issue if you ever have to rip one out to work on the wiring. Better to have to do that and have a waterproof outlet than to risk water damage.