I am guessing some mix-up in the way it is all connected where somehow the solar is opening a breaker or switch
AFAIK the dash light does not indicate over-voltage, but indicates no charging is happening at the engine battery. With the light on, you could check the engine battery's voltage, turning the headlights on.
Do you also have a dash switch for using the house batts to start the engine? My class C does not, only has the isolator with ignition run to it. Some have a three switch affair for that, where the run circuit to the isolator gets there a different way. Don't know if that could be in backwards.
Maybe something is on a buss that shouldn't be with the others on it.