The standard 14-15 amp dynamo is not enough for your light/cab setup.
Go to your dealer and have him order a B7312 high capacity alternator option. As was stated previously, its $165 + installation which takes a few hours.
Somewhere here on TBN is a discussion on how to do it yourself since it is more than just replacing the dynamo with an alternator. You have wiring harnesses and a new dipstck to install too.