I have not heard of an "emergency start" button. How does it work?
As others stated, my RV has a switch on the dash that allows you to gang the house battery to the engine battery so that you can start your engine using your house batteries if the engine battery is dead. This is a momentary switch you have to hold while cranking the engine and it ungangs as soon as you release.
If you flip the switch the other way, it gangs them together one-way only so that the engine can charge the house batteries.
Unfortunately if you forget the switch is flipped that way and you run your generator and start up the engine at the same time you will fry the voltage regulators in the car and RV as you have competing voltage inputs.
Steve