How to use routing and remote access on server 2008 DC for an internet on isolated DHCP enabled LAN network with one IP having internet connectivity?

You would configure the router like normal. It has no effect on the Windows Domain. Disable the DHCP on it and have the domain controller do DNS and DHCP. Point the clients at the domain server for DNS. The domain controller will use the root servers unless you specify forwarders. All the internal clients and servers should use domain DNS.

While you are configuring the Routing and Remote Access Service you will have the option of either specify a range of IPs to be assigned to the Remote connected clients, or specifies your DHCP server within your local network, so that the DHCP will assign IPs to the Remote connected clients.