This type of thing happens on occasion and is a result of the foam insulation used. The problem is that when manufacured the foam amount was a little too great and since it expands there is an amount of pressure built up on the back side (in your case) of the liner. about the only thing you can do is to carefully do as RV Roamer suggests; drill a hole at the end of the crack. You can aquire a white vinyl tape to cover the crack to help hide it. If it is still in warranty contact the manufacturer and request it be taken care of. Even if it is out of warrranty they may send you the tape needed to cover up the crack.
Mike, RVIA & RVSA Certified Master RV Technician
Amy, Dr. Assistant - Roxie & Mei Ling, four legs each
2000 Gulf Stream Scenic Cruiser 450 hp & 1330# torque
06 Saturn Vue, 06 Chevy Z71 4x4 & 2014 Corvette Z51 M7