Update:
Cut the wires to the Magnetic Reed Normally Open Magnetic Door Switch. Stripped and wire nutted the ends together. Ground circuit from the 4-pin connector is still OPEN.
Off to check wiring...
BROKEN SPLICE IN JAYCO'S WIRING. They used an oversized butt connector (for the sizes of wire being connected) to join the grounds for two little LED step well lights, the Magnetic Door Switch Ground Side, and a ground wire to the Chassis. Going back now, to cut the butt out (can I really say that here...?) and test the ground back to the chassis. If good, then all I have left is to rebuild that 4-way splice and re-connect the Door Switch wiring that I cut earlier today.
That'n pick a lot of dirt out of my hair, teeth, eyes...