Metal duct work is not necessary but would not hurt anything if it were. Just follow the installation insturctions as to the clearaneces for minimum and maximum spacing and you will be fine. GOod luck and let us know how it turns out.
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