Rebuild kits are available...but the only parts are what Thetford has left in inventory, which is not much. And a retailer may already have bought the last of those.
Give Jock Allpress ("JockO") a shout...he was with W-C for decades...the only "go to" guy left for W-C. He's my guru there.
His cell is 401 225 4529 ... email jallpress@thetford.com.