Garmin RV 770 user here. I travel a lot (30,000 miles a year) and love it. Don't have to worry about having cell signal and don't have to remembering to download maps ahead of time. I like that it avoids low height and weight roads. I have lifetime map updates, seems like the updates are available about every 6 months. The only downside is the traffic feature doesn't work as well as google maps on the phone.
I set the Garmin to the destination for the day, and have free use of the phone. I'll use the phone to navigate to stops during the day like fuel and rest areas.
If you're on the move a lot, I'd recommend a dedicated RV GPS. If you just travel occasionally, you're probably fine with just a smartphone.