Thanks for the input.
I have an E20, purchased new in the late 60's. The problem has only occurred recently and I have replaced the reverse contactor with a known working one and I have checked all the wiring for continuity. The power use gauge shows a slight increase, but the fuel level doesn't change.I suspect it might be something in a circuit board.
As the batteries become drawn down, the reverse gets weaker until it is almost non functional.