First, it appears that turning off everything may not actually be turning everything off; especially if there were any dealer installed options.
You can either figure this out yourself or have the dealer do it, which will take awhile. At minimum, if you have the electrical skills, you can at least verify the issue.
Get yourself a clamp-meter, such as the B&K Precision 316. The reason I recommend this meter is it can measure down to I think 1mA DC... at least it's a lot lower than the typical clamp meter.
The accuracy though is only 2.5% which is not that good for a meter, but about standard for clamp meters.
What you do is turn everything off, then clamp the battery leads, one-by-one and see if you measure any current. That will tell you if something is discharging your batteries. The beauty of a clamp meter is it is fairly safe, and you don't have to disconnect heavy battery cables.
A standard clamp meter will work as well, as long as it reads DC Amps. But it won't likely have the resolution of the B&K. The B&K is designed for small currents, while the standard meters are designed for industrial/residential household currents.
If you suspect your engine alternator is not charging the batteries, on some Thor coaches (mine), it uses a combination of an Intellitech Isolator Delay Relay/E, and a Trombetta Contactor (but not all coaches charge the house battery via the engine). Thor may use others as well, but this is what was on my coach. Here are links to the documentation for these two items:
These components may or may not be installed in your coach.
But you can verify the operation of the engine alternator circuit - whether or not it is charging the house battery, by putting a voltmeter on the house battery, and watch the voltage as you have someone else start the engine. If after about 5~10 seconds you see a tiny "blip" of a voltage increase (around a volt), the engine is charging the house battery.
Or if Thor did not use a Delay Relay/E, but just the contactor, you should see an immediate increase in voltage when the engine starts running.
If you don't see the small voltage increase, then the engine is not charging the house battery. Again, it might not be outfitted to do so, which is something you would have to investigate.
The 5~10 second delay is from the Intellitech Delay Relay/E, as it gives the alternator a few seconds to stabilize it's output before connecting the house battery, which is done by the Trombetta Contactor (the output of the Delay Relay/E is connected to the Trombetta Contactor).
Often, when the generator has not been started for awhile, they can be awfully hard to start. But make sure you "prime" the generator. You do this by depressing the ON part of the start button for just a second - not long enough to start the generator. This wakes it up. Then you depress the OFF part of the start button for several seconds, until you see the LED light up. This primes the fuel pump. Then depress the ON part of the start button, and the generator should start a lot quicker.
You could also have a bad house battery. They install about the cheapest batteries you could find (would you expect anything different), and if the factory or the dealer has neglected to put a charger on them periodically while they are sitting around, and if they sit around for an extended period of time, they (especially cheap they) can sometimes go bad.
Hate to say this, but your engine battery could also be going bad. Again, if the coach has been sitting for awhile, I would not expect the factory or dealer to do any preventative maintenance on the batteries.
Of course the factory and dealer will finger point to the battery manufacturer, and the battery manufacturer will finger point back saying proper maintenance was not done thereby voiding the warranty.