Welcome to the forum! Hope you enjoy every mile and every night in your new coach.
What I did was take the cup holder insert with me to a couple of stores until I found cups that fit properly and bought new ones especially for the RV. There’s a nifty Yeti coffee cup that fits perfectly for me and a no-name insulated big water bottle. Many folks try to fit plumbing parts or something in the cup holders to make them fit a favorite cup but that seems harder than it needs to be.