The WINS & DNS values from the external DHCP server will never be passed through the ASA to the L2tp client, this is by design.
You have to configure the WINS and DNS address' in the group-policy.
ASA(config-group-policy)# wins-server value
ASA(config-group-policy)# dns value
In this example, the IP address will be assigned from the DHCP server
and the other address's will be passed down from the ASA.