In 16 years of full-timing we rarely made definite plans except if we were to meet up with someone at a particular time or wanted to go to a particular function. We had a general idea for 3-4 months out and headed that direction but no reservations. For instance, in summer we'd normally cover 3-4 states.
90% of our stays are at public parks and even with our 40' motorhome we rarely make reservations. We've been to all the major ones in the west. We get there early in the week and early in the day, many times spending the previous night outside the park but nearby so we can grab a site as folks are leaving in the morning.
For mail, we use Escapees RV Club mail forwarding service. We email or call them and our mail has always been received in 2-3 days as a small-town post office - general delivery - even in Alaska. We've never found a post office that wouldn't accept it. The key is to pick a small town. If we're heading toward a particular area that's the post office we'll pick. We've never made advance preparations with a post office.
For appointments, such as doctors, we make them when we'll be in an area a longer period of time. Winter months have worked the best for us as we usually stayed in the same winter area year after year so that's where we established doctors. Knowing that's where we'd be, we'd call for appts. approximately 3 months prior.
Everything is doable for full-timers with minimal planning.