Condensation on HVAC Ducts in Basement
One thing to check: measure the return and supply air temperature inside the ducts. A rule of thumb is a temperature difference of no more than 20 degrees. If it is more than 20 degrees the air delivery is too low ( you need more air flow ). Yes, the trunk lines and all the supply duct work in the unconditioned space should be insulated since you cannot control the humidity in that space. Adding insulation to the trunk ducts will do very little to change the efficiency of the overall system. Since You have a single return grill which you said you had in one of your answers, then all return air from rooms with doors will be from the space at the bottom of the door. If that gap is too small air delivery will be reduced. Think of each room with a door as being a balloon; you can only put in a certain amount of air. I would insulate the duct work and use insulation designed for that purpose available at some Home Depot, lowes or hardware stores and measure the temperature difference in the ductwork.