Follow the shore line into the rig, it should go into the transfer switch. With all power off/unplugged remove the cover from the transfer switch, should be 4 or 6 screws. Once the cover is removed you can mark the wires and remove them all three sets. Gen/shore in and switched power out. Remove the switch and take it apart. You may beable to clean the contacts with some emory cloth/or light sand paper. Clean it up good with some alcahol (Not Beer
) and let it dry out, you can also use canned or compressed air to help clean and dry. Put it all back togeather and see if that helps. From what you said, the shore power contacts may just be sticking. Sure hope this helps. Also depending on what you can see and reach you may not have to take it out to clean it. good luck and keep us posted.