Charging the truck battery with solar- will a 1.5W panel do?

If they are draining to where they won't crank the truck after a few days without the TC plugged in the batteries are failing or you have a pretty significant parasitic draw (short somewhere). The batteries just sitting in truck should last several months. Start chasing down that short

To answer your question directly - for large batteries use 10watts per battery if not using a controller.