Brake lights on all night will do it.
Other thought is that somehow your coach battery is connected, while motor off, still to your starting battery, and that the coach battery is of low voltage and robbing your good starting battery of its voltage. Check coach battery, if it is fine then this is not the case. If so, then check the connection between the two - it will either be a solenoid (looks like a small shiny steel can), or a battery isolator (looks like an inverter or stereo amp, likely blue color). I've never seen either go bad though.