RV outlets commonly are self-contained wiring devices (meaning they don't have a separate electric box). If you have access and space and wire slack to install a box and a traditional outlet, it should be relatively straightforward to put in one with a USB charger; but lacking any one of them and it would be a rather tricky job. I haven't happened upon any self-contained ones with USB chargers built in, though I must admit I haven't really looked either.
Something like
this that mounts onto an outlet might be more practical. You wouldn't be able to use the securing screw, most likely, since the self-contained outlets don't have a central screw hole for a cover plate.
As to your actual question, I don't have any personal experience or particular recommendations.