I think dash-mounted GPS's are becoming obsolete. There are so many smartphone/pad GPS's available, many for free, that have regular map updates performed automatically. And they seem, to me anyway, superior to GPS's.
On my most recent 2,000 mile trip two weeks ago, I tried out three of them: Google Maps, MotionX and GPS Copilot. I liked Copilot best. It loads the entire US in so it doesn't have to access the internet at all while traveling. Only once for just a few miles, was there an issue with recent road work leaving me driving "no where" on no roads. All GPS's do that from time to time.
I would like MotionX best, except that it wants near-continuous access to the internet, which isn't very feasible. Except maybe around big cities.
I was using all of them on an iPad, btw. My phone is too small for my old eyes, but the iPad is great. Oh, and it has a car/truck/RV option that lets you establish height limits and specific road preferences. If you want verbal prompts, which I don't, then the 'free' GPS costs, like, less than $10 per year. And again, all updates are free.