Before extensive RV travel years ago I checked into this and decided to use original Medicare with AARP supplemental plan F and now plan N and i did receive full coverage for everything throughout the western USA and no or small copays and NO NEED FOR REFERRALS FROM A PCP.
I also have full medical coverage to include dental and eyecare from the Veterans Health Care System but perfer private medical for serious issues such as cataract surgery, etc.
My research showed that MA plans were to limited and restricted for nationwide full time travel.