When the iPhones first came out, I was forced to switch to AT&T. What I discovered was that as long as you stay on the major thoroughfares and in or around big cities, you are fine with AT&T.
Now that the iPhone works on all networks I have switched back to Verizon. I find that Verizon has much better coverage in fringe areas. (e.g. boon-docking).
On the other hand, I also have bought a WeBoost RV 4G. It is an amplifier that boosts all the cellphone frequencies. It requires one to put an antenna on the outside of your RV pointing at the cell tower (There are websites for that). I have used it several times to make sure that I have a good cell signal.
Added: HWH Kick-Down Leveler; RoadMaster Anti-sway bars; Tornado Rotary Tank Rinser; Accede Backup Sensor Kit; cylinder locks (w/one combo lock); custom sink cover; Kenwood DDX 5706S front radio; Pioneer MVH-X5560BT rear radio; Oxygenics Shower head; ScanGuage II; Blue Ox Tow Bar w/RVi Brake 2; Winegard Rayzar Automatic TV antenna; Trimark Steel door lock; Dometic 310 toilet; whatever is next.