I've never tried playing movies via USB on the iRV, because I know it won't support the format of a lot of my movies. I know from reading that it's very picky about both the format of the USB storage, and the format of the movie file itself (codecs etc).
I'll be taking a USB harddrive with us on our next big trip, and using the Playstation (which we just use to watch blurays) to play the movies in the main living area, and the samsung bedroom TV for if we want to watch any in the bedroom.
2015 Rockwood Signature UltraLite 8281WS
2016 Ford F-250 Lariat CrewCab, 4x4, 6.2L V8