I hope you've found a way to repair your roof by now. The only really good way is to replace the roof assembly. Most of those ABS tops did or will do what yours has done. You can patch the roof with some white Parrbond in the cracks, and try to find a newer top to install on your trailer. FTCA has told me they are no longer making those tops. You may be able to adapt a newer one, just make sure the lift supports and main door are in the same spots.
Two wrongs don't make a right, but three rights make a left.