Lonnie Dealer Agreement

I am assisting my father in fixing a situation at his mobile home park. My 80 year old dad has a “handshake” agreement with a lonnie investor for 11 of the units in his 96 pad mobile home park. The investor is behind on the rent for all the units. I would like to get a written agreement with the investor to clear up the agreement and provide additional security against the mobile homes in the event of a additional future default of payment of the rent. Does anyone have a agreement that I could use? Any additional comments or thoughts would also be greatly appreciated. Thanks in advance for any assistance. Steve