In SQL Server, the schemas are associated with users. Each user has atleast one default schema. You can associate the tenant with schema, that means you will have mapping for
Tenant -> Schema -> User
Dynamically construct your SQL Connection string and pass the SQL user corresponding to that tenant. This way the default schema is automatically pointed out and you Dont have to change your Dapper queries and dont have to write dynamic queries as well.
Note: Don't forget to check the Connection pooling implication.