I suspect you will have to go through a dealer but Thetford do sell spare parts - more from...
info (at) thetford (dot) com
www.thetford.com
101 Jackson Road
Ann Arbor, MI 48103
Phone: 1-800-543-1219
734-769-6000
Fax: 734-769-2023
Mailing Address
P.O. Box 1285
Ann Arbor, MI 48106
Peccavi, August 2009