The valve is held to the tank flange by 4 stainless steel screws. If you remove the 4 screws the valve will separate form the tank. Between the valve and the mounting flange there is an O ring. This O ring is one of the set that comes in a standard rebuild kit form Thetford. The kit runs 20 bucks. There will be the seals to replace the internal seal in the valve itself as well as a gasket.
The plumbers putty may work, but the long term fix is to get a new seal kit. the flange should be glued to the tank outlet. If this is where it is leaking you have a bigger problem to worry about. Namely the issue of IF the tank itself is cracked or just needs some more glue