Oh, I see, a bus converted into a motor home...
Visit your RV parts place and discuss a device called an ISOLATOR. Now one word of warning: If your bus uses a 24 volt system, disregard this, the bus must use a 12 volt sytem for this to work.
The isolator (There are two types and several sub types) is either a RELAY (Solenoid or contactor at the kind of current we are discussing) or a Diode box (I recommend the former)
A diode box is 2 or 3 "one way" valves for electricity, they let alternator power flow TO the batteries, but do not let power flow battery to battery. They do, however, introduce voltage loss, so the 3rd one-way valve is for the voltage regulator, Many alternators today the regulator is internal and these will nto work well with a diode box.
The solenoid is an automatic switch, IT usually has a delay, You start the engine, the timer counts down and CLUNK, the batteries are now in parallel.
Some of them have a bi-direcitonal system.. You plug into shore power, the house batteries charge and when they are near full... CLUNK, the batteries are parallelled.
This is the best kind, called a BIRD (BI Direcitonal Relay Device) as 2 weeks after you pull into the campgroudn you go to start the bus back up.. and it starts.
Home is where I park it!