Agriculture rtc validity

RTC is primary revenue document it shows who is holding and cultivating the land. 

Mutation is a document which record all changes of ownership transfer.

Better you should have all the revenue documents including tax paid receipts, survey records and other antecendent documents of title under which your father acquired the ownership.

First instance send a legal notice then file a police complaint and approch the civil court and get the restraining order from the court and prove the title with possession.