No DC Power after shutting down generator

Some RVs on AC use the converter only for 12V house power and switch to the battery when the AC is off. Don't overlook a open ground. Use a voltmeter connected to frame ground to check voltage and not some groiund wire that may not be connected to the frame. Unplug and disconnect solar to so that the battery is the only source. Now turn the lights on for a load and start checking from the battery downstream and you should be able to isolate the problem.
A bad battery can also be your problem. Voltmeters use very small amps and hence voltage OK for voltmeter but fail with a load, hence have the house lights on.