newmar 2005, engine wont crank over and no voltage

Lights take very little current. Starters on both your generator and engine require a lot of current. If the battery terminals are tight, check the other end of the cable at the ground to frame connection. A loose connection will still allow the lights to function but might not allow enough current for the starters, especially if the lights dim, or go out, when you turn the key to the start position.