First.. Call Thetford and find out the material the tank is made from and be very specific.
Go here and order this kit BUT they sell different types of plastic rod to match the plastic of your tank. They do sell a complete kit with small amounts of most plastic types but it's about $160. Buy the smaller kit for about $60 and the plastic you need for about $20 and save some money.
Shortened link to Arizona Tools..
http://tinyurl.com/54h6mv