Answer:
Typically, they are not. The mortgage contract the tenant has with with the lender is between the tenant and them. The rental contract you have with the tenant is between the tenant and you, and the lender does not enter into this picture. These are two separate transactions.
Insofar as any unpaid back lot rent owed by the former tenants before the foreclosure - the lender is not responsible for these and you would have to file a personal lien against the former tenants. However, you MAY have some leverage to hold the mobile home for the payment of "storage fees" incurred since the date the lender foreclosed before you allow the home to leave the park.