hderr: I think you are asking the mechanic a "hypothetical" question. I.e., all the mechanic can do is check the voltage of your house batteries when the engine is not running, and again with it is running. ...And anytime you see 13+ volts to your house batteries, then you know your alternator is changing; or if you have solar and are in the sun then that explains why you voltage is above 13V.
If you have a battery charging circuit installed, and you think it's not working, then you will have to prove your concern before you "pass the buck" to your mechanic.
My guess is that you are not happy with the SOC your house batteries have when you reach your camp spot?
...You do know house batteries can only last 1-2 years in some cases? ...How nice are you to your batteries? I.e., do you start the generator when your battery SOC drops to 50% or do you run them down to 11.8V?
==> When you store your coach, do you use a battery maintainer?
Maybe your house batteries are very sulfated? ...If so, they can measure good voltage, but not pass a load test.
==> Before you question your charging system you really need to verify your house batteries are in good shape!!! ...And then, I think you will find your changing system will work to your expectations when you replace your house batteries.
ALTERNATE SOLUTION
My coach did not come with a house battery charging circuit, and in many RVs the house battery charging circuit fails.
Are your house batteries located next to your engine battery?
Solution: Buy a $85 Keyline Voltage Sensitive Relay (VSR), aka Battery Isolator, and disconnect your stock house battery charging circuit if it's shorting out, and then you will always have 100% full house batteries when you get to your camping spot. (Easy install you can do yourself.)
https://www.amazon.com/Dual-Battery-...xpY2s9dHJ1ZQ==
One way to do a rough study on "how good" your house batteries are, and this test is sometimes better than relying on a load test device, is to do this:
* Start with what you think is 100% SOC and 12.7V or better on our house batteries.
* Boondock and run a few accessories. If your voltage does not drop very fast, then run you microwave to heat up some water for 2 minutes. And if your house battery voltage is only drops by 0.1V then you probably have good house batteries. If not, your inverter will shut down or you voltage will drop much faster than normal.
Last but not least: Don't expect the mechanic to check your charging system for free! Especially at Winnebago, Forest City. I have been there a few times and they charge for every minute your RV is in their shop!
So while you are probing for answers at WBFC, IMO, you have more homework to do on your own first.
Plus, you will not be able to argue your way out of the bill at any diesel shop, no matter if they fix your problem or not! This is the trucking industry for you! Time is money!
A better way to go is to stop by a Les Schwab or Interstate Battery Store who may test your batteries for free. ...Even some Walmart Stores will help you for free. But any diesel shop will run the meter on you!
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