The first thing I recommend for anyone considering long term RVing is to sit down with your current primary care physician and see if she/he think you have any issues that might be a problem.
As for access to doctors and facilities - a lot of people live most of their lives far from the level of care available in a major city. I grew up in Arkansas where even today if it is something really serious, people go to Little Rock. Often a 3 hour or more drive. My BIL has had to make such a trip several times.
Most of us do like to have a primary doctor somewhere that we visit once or twice each year. In the major snowbird areas, there are many excellent doctors who are used to working with long distance travelers.
We are mandated to use a mail order pharmacy for our regular medications by our insurance. However, we use Walmart pharmacies for anything new or short term. They are not the fastest, cheapest or easiest to get to sometimes, but there is one almost everywhere in the US.