From what I can tell, the 600 watts are audio output, not power draw. Wouldn't this wattage be based on the voltage supplied to the speakers which is constantly changing?
Personally, when I want to know what something is drawing I simply measure it. Get an inexpensive volt/ammeter from Harbor Freight and hook it in series with your positive battery wire. Keep in mind that these cheap meters are for measuring small currents so make sure everything in your camper is off when you hook it up.
Next, with the meter switch on "amps" take note of the reading when everything is off. There will be some parasitic draw from things like the radio memory and CO detector. Now, turn on your stereo and you can watch the amperage on the meter change as you adjust the volume. Subtract your initial reading of the parasitic draws and you will then have an accurate reading of exactly how much your stereo is drawing.
You can do the same thing for other appliances in your camper such as lights, fans, furnace, etc. Just don't turn on anything like a 12V refrigerator (other than the display board when it's running on propane).
Having said all that, I have a 130 watt solar panel on my roof an I never run out of juice--just be careful with that furnace; perhaps run it sparingly when getting ready for bed and in the morning.
Sorry to be long winded, hope this helps!
Bob
