This switch can be used to provide emergency starting power from the house batteries if the chassis batteries are dead. Press and hold while turning ignition key for emergency starting power. This switch will add the house batteries to your chassis batteries for additional power. The best procedure, as described by Coach-Net, is to: 1.
Source: http://www.myrv.us/electric/ The RV owner should know the difference between the 30 and the 50-amp RV service also the other 30 and 50-amp Services that are used in everyday applications but could be harmful to the RV. Most importantly everyone should know how to test a receptacle BEFORE plugging it in the first time! If