This is my planned install for the 2nd controller and a 2nd panel to my camper. The two controllers will be connected to a bus with no fuse.. Is a fuse between the controllers and the bus needed If I already have a fuse between the bus and the battery?
Your controllers and any other charge system should be all individually fuse to the bus. Otherwise you have to use cable which can take maximum DC current of the system, also to protect the controllers.
In my camper i use 12V circuit breakers in all the positives of the mppts and dc-dc charger. Prior to connecting to the main bus
Overcurrent protection is necessary whenever the source of power can exceed the wire's ampacity. In most DC systems, charging sources will limit themselves to a safe current so shorting a wire from one of your solar controllers won't cause excessive current in the wiring between it and the battery.
However, the battery can typically supply more current than the wiring can withstand, so a fuse close to the battery is required. In your situation, the fuse between the battery and the positive bus bar will protect the wiring between these two pieces, but may not protect against a short in the wiring from the bus bar to the solar controllers. These will typically be sized for the current the controller can supply to the bus bar. So additionsl overcurrent protection is needed at the bus bar end of each wire to the solar controller so that if that wire shorts to the negative lines (or chassis) the fuse will blow before burning the solar controller output wiring.
