I looked up the specs on 2016 Minnie Winnie on Winnebago's site. Here is what it said under Cab Conveniences:
...
Radio w/AM/FM stereo, CD player (NA 31H)
Radio/Rearview Monitor System 6" LCD touch screen color monitor, remote, AM/FM stereo, iPodĀ®/MP3 input, color rear camera (31H)
...
Her unit may not have a USB port in the cab, but it looks like she could play music (or audio books or whatever) via an MP3 player or and iPod. That might require a cord.
Can she send you a photo of her sound system? Or a photo of the appropriate page in the owner's manual? Then you will know what you are working with.
My 2012 daily driver car has a cord with a USB port on it that goes into the glove box. I plug a flash drive in there and can play almost endless music, no ads, no static. It is wonderful. Sadly, my older RV truck chassis does not provide such delights.