Starting motor automatically switches RV to "Aux Battery", and then drains the battery

[JeffAndPam]

Our car battery kept getting drained, and it took a while to find out why.

First, on the Panel pictured below, in the top right corner I have the ability to switch to "AUX BATTERY", which is the car battery.
When I put a volt meter on the negative terminal, I notice 10v being pulled.
When I go to switch this OFF, I can actually hear the audible switch occurring. (BTW, the LED below the switch never comes on). I also notice the voltage pull on the car battery drops to almost 0.

THE ISSUE: I found that whenever I start the RV, it apparently automatically switches on the AUX BATTERY switch. (Since the LED wasn't working, I was unaware of it until recently). After starting and shutting off the engine, when I go hit that switch to OFF, I hear that switch occur. If I ever forget to do that, I get a dead battery, even when connected to shore power.

Reading through the manual, I see the following mentioned:

The main and auxiliary battery disconnects are automatically
engaged anytime the ignition switch is
turned on.

That seems to confirm what I'm experiencing. But it's draining my car battery every time I forgot to switch it to off. I can't see that being an original manufactured "feature." Am I missing something?

 

[Diesel Pusher]

I assume by "car battery" you're referring to motorhome chassis battery? If yes, it's a Fleetwood OEM set up to turn the Aux battery switch on as soon as engine starts. Mine does the same thing. You didn't mention the converter/inverter system your coach has, but it should likely maintain chassis battery via the converter when plugged into shore power. When not connected to shore power, and with both battery switches turned off and you come back to a dead battery, you have a parasitic load somewhere.

 

[JeffAndPam]

I assume by "car battery" you're referring to motorhome chassis battery?

Correct: referring to the chassis battery (vs the "house battery").

What's odd is that this draining occurs while connected to shore power. My owner's manual says:

Power Converter
The CD power converter will charge the chassis
and house batteries when plugged into 120-volt
AC service or by the generator (if equipped)

I was connected to shore power for several days once, and went to start the engine and the battery was dead.

Before I get too many replies, perhaps I should try to do more diagnostics so that I can explain in better detail what's going on. I value forum posters time, and want to do ensure I've done all my homework first.
I'll try things like: Testing DC current on the chassis negative terminal when connecting to shore power vs not. Also checking DC current on the house battery.

So here's a question: When you turn on the engine, it switches this connection on. But is it suppose to switch it OFF when turning off the engine? What's the expected behavior? I'm just wondering why I have to remember to switch it off each time I turn off the engine (or disconnect the battery negative terminal). I don't see that discussed in the manual.

 

[Diesel Pusher]

You need to turn both battery switches off, they're not automatic. The inverter, smoke detectors, Co2 alarms, dash radio and TV will suck power from the house batteries when left on. When the house batteries get low, the alarms etc will draw from the chassis battery....... in some cases. If it's only the chassis battery dead after being connected to shore power, start with testing the converter/inverter

 

[TheLooks]

If the convertor checks good, there is a fuse or bad connection between it and the battery. If it were turned off, I would expect the battery to not charge but also to not discharge. You may find something amiss around the latching relay that the switch actuates.