We have the 'X', which was their top model when we acquired it. It has very rarely failed from getting some sort of signal in places where none was apparent on the phone's indicator.
The interior antenna, which is suppose to work wirelessly within a number of feet around it, seems to work best when our phones are practically touching it. Therefore, we let it dangle from over the dinette table and that keeps it in 'the phone booth'.
When signals are really sparse or not shown, we attempt to just send texts back and forth. They sometimes make send/receive at a much later time than when sent, apparently getting a momentary connection as weak signals come and go.
As for the outside antenna, as airplane pilots say, "Altitude is your friend.". Getting that as high as possible and away from other metal objects on the roof of the camper is truly your friend.