I finally remembered to get you some pictures of my 1989 MH. It sat for 8 or 10 years and needed an injection pump, injectors, turbo, air receiver and of course tire$ and batterie$. It appears you are luckier than I was.
One picture is the front of master disconnect switch and the other is the back of it. The red knob controls it. The other picture is the back of it. I took the picture of the knob with the camera resting on the left front tire and based on your pictures this is about where the solenoid you posted is located. Like Vlad posted the other day, the disconnect kills everything