If I ran the generator then I would get full voltage to the house batteries but in order to start the generator I had to start the RV.
Curious, doesn't the generator just supply 120 V to the converter just as the shore power does? Why the difference on the DC side then? If you are measuring the voltage at the batteries with the engine running, maybe the voltage is provided by the engine alternator, not the generator/converter. You could find out by starting the generator, then shutting off the engine and measuring the DC voltage at the batteries.
The difference in voltage at the cutoff switch vs at the batteries could be explained by the switch being open or broken open.
I know it is all very confusing with wires all over the place and no way to tell which is which. It would be nice if the wires were labelled so one could get hold of the engine supply wire at the batteries, disconnect it and measure just it's voltage. A DC clamp current meter is very helpful for seeing how much current (charge per second) is flowing through which wire.
Anyhow, understanding will come from isolating each source of DC so it can be checked independently.
