I'll let Gerald tell you in his own words which make more sense then mine but basically it's an Isolater Relay in your rear fuss box in the engine compartment that controls the flow of charge to your house and chassis batteries.

can't type either, that would be fuse box, not fuss box.

A Big Boy is the name of the battery pack combiner solenoid system that is used on latter model Monaco built Beaver coaches to combine the battery banks when one battery bank is charging in order to charge both banks at the same time.

Gerald

ALL newer BEAVER and MONACO coaches use different system, than older BEAVER COACHES.  They use a INTELITEC system. which uses a Bi-Directional Isolator Relay Delay-Diesel 2.  This system has a lot of advantages. You can down load a service manual at http://www.intelltec.com.   1. It doesn't need an Echo Charger.  2, You use  a standard  ALT. and
you don't use a Bat Isolator.  Check it out

ROY MUELLER