You cant, he drilled put the corners of the cracks so the will not spread. The only permanent fix is to remove the abs from thw core and coat that with grizzly grip or bedliner. Anything else is a stop gap and will mostly by you time. Once water gets under the abs its kinda game over. The water heats and cools expanding and running under the abs causing more cracks. This is also dependent upon exposer. The roof will not leak with the cracks as the foam core is intact and will not let water in. Search the site for abs roof repair, plenty of people have tried it( me included), and have had various levels of success.
I screwed down the outer ABS skin where the roof was delaminated.
Sjm - Here is what I was referencing. I wasn't too sure if that was possible, as you and others have educated me in my other post, I wasn't sure it was feasible to run screws down through and possibly compromise the watertightness of the foam. It's very possible I misinterpreted the post. As I'd stated, some of the plastic is coming up and forming "ridges" on the roof, though it hasn't gotten too bad yet. If there was a way to smooth it all out I'd be willing to try. Thank you, I do appreciate you and others passing along knowledge of how these campers were designed and helping me learn.