Using a stand alone smart phone as a GPS

I still have a dumb phone but have been reading threads on here about how folks are using their smart phones(I phones and Androids) and downloading the Co Pilot APP for $10 and using it for a GPS. From what I understand the Copilot app lets you customize the RV height and wt like the RV specific GPS's. This seems like a much cheaper solution than the Garmin 760 or the Rand GPS. My kids are always buying new phones for the latest technology so my question is can I use their old I phone 5 model as a standalone GPS in my MH with the co pilot APP and not use it as a phone without going through a phone provider? I do plan on getting an I phone for my wife and I will keep my old flip phone. So my next question is when you buy an APP can up use it on multiple devices or do up buy one for each device?