Every class C cabover bed with seamwork in that area is susceptible to water leaks. If I really needed that extra bed, I would get a seamless bucket design. New seamless ones are very hard to find on the large Ford versions, but the smaller Sprinter versions are readily available.
If you don't need that extra bed, then I strongly advise to consider a B+ as they are both seamless and aerodynamic offering improved handling as well as fuel economy. They are quite nice inside as well, offering storage and entertainment along with providing a nicer more open living area.
If you intend to take full advantage of the massive storage a conventional class C cab-over offers, you are at risk of over-loading it with too much weight which will further encourage seams to split.
I imagine it would be heartbreaking to find water and mold damage inside a motor home. This is the time of year for owners to head out to their storage lot and discover their wonderful class-C has been leaking all winter long. I don't know the percentage of failed seams, but I would not risk it if I did not need that bed in the first place.
Here is an example of a hard-to-find Ford chassis seamless cab-over bed.
This is ideal.
Here is the same brand motor home, but with cab-over seamwork.
This type of seamwork is not as bad as some.
This design is most susceptible to water penetration.
Again, the same brand motor home.
I am not picking on Tioga here. Actually I commend them for offering a seamless cab-over model. Most companies do not.