I touch on mail a bit in this article:
Camping is fun! Why not go fulltime? | Learn To RV
But basically, most RV parks allow packages to be delivered (especially medicines). If you’re somewhere that can’t/won’t receive them, you can use General Delivery at the nearest post office depending on shipping method. For UPS, you can sometimes have shipments sent to local stores. I assume FedEx is similar.
For our non-regular prescriptions, we just use Walgreens.