For many years I've used a Garmin on the dash never had a GPS reception problem. and that includes CA, OR, WA and other westerns states with numerous dense trees.
The Kenwood Radio/GPS in my MH with it's roof top antenna is CLEARLY Garmin based. And it's information is CLEARLY the same as my Garmin word for word with it's roof antenna not any better than the dash Garmin regardless of direction headed or trees etc.
Dash Pro:
It's always there, just turn on the ignition
Doesn't get left in the car or elsewhere
I'm thinking about other pros
Dash Cons:
You have to turn on the ignition to view or update
Can't move it to a car as desired
Can't bring it inside the MH or house for:
Adding points of interest
Viewing a route
Updates are generally easy, just pay the dealer
You can't leave it at home
The battery provides hours of operation anywhere, err many hours but then you have to have the ignition on
I can download and update my portable Garmin maps, etc (yes very LARGE files). Did I say I can do this vs the dealer?
Our recent Grand Cherokee has a built in GPS, wish I could transfer my many years of destinations to it and the MH. Maybe it's somehow possible but I'm not planning on those adventures...
Of course a DIY install could change some of the above.
