I repaired mine at a campground in Fl 3 or 4 years ago. As Bounder Law said, Thetford has good repair manuals online. If it's like mine you'll need a socket on a long extension and probably a swivel adapter. Under the seat is plug that can be removed. That gives you access to the rear hold down nut. Front one, don't recall how I accessed that one but not from above like the rear one.
The valve assembly on mine was gunked up with mineral deposits. I was able to clean it out, replace the o-ring, grease it up and it was good to go. Caution, the valve assembly is tricky to get the parts lined up while reassembling. Feels like something is missing or worn out. It's water pressure that seals up the rubber plunger/seal. If you're not comfortable doing that you may want to order a new valve. I lucked out and it worked but who knows.