1985 southwind multi battery set up?

The majority of older high end RVs that I do still do some work on are connected exactly as your diagram shows. The diode isolators are reliable, simple and extremely easy to diagnose should one fail. The solenoid in your picture is usually connected to a single switch that is often momentary so the hotel and main engine battery can be cross connected to boost either in the case that one got drawn down too far. It is such a great convenience that when one does fail, I get called right away. Many owners have added "combiners" to cross the hotel and main engine battery when there is charging voltage present and this is effective, but when the device fails, I have great difficulty giving an owner instructions to diagnose as it is not a simple test. Most combiners cannot pass starting current and so are of little value if you need to cross the hotel to the main to crank the main engine. This is exactly the same work that I did on other peoples expensive boat before the depression hit here.

Matt