I have a 2000 Diplomat and sending unit went out also. It is located on side of tank. It is accessible thru a white screw plate in rear of first compartment on passenger side. It is supposed to be a 100 gal tank but when measured it is a 110 gal. call centroid with meas. and they can replace. You will have to empty tank because it is located on bottom side. It is a diy project.
http://www.centroidproducts.com/moncal.htm