I have little faith in that little switch by the driver's left knee. I could see it being useful if accidentally draining the chassis battery. But to sit and hold that switch for a half hour while the generator is running to get the battery recharged, does not make sense.
I carry an extra-long set of heavy gauge jumper cables for jumping the chassis battery with the house batteries. I have used them when my original chassis battery was at the end of it's life. Until I was able to replace that dying battery, with every additional start, I had to increase jump-start capacity, first by running the generator, and later adding my external 40 amp charger. Between the three different sources, I got the engine started that final time.
The jumper cables are also handy for the tow vehicle and in helping other people.