Understanding step down converters

[Lazarus]

This is not about the machines but there are a lot of members that deal with electronics.
Trying to adapt 18 to 40 volt batteries to be used in an rv style system when normal system drains down and can't be changed. As a contractor I have batteries for power tools. Want to be able to slip one in if needed.
A step down converter that state's 8 volts to 40 volts step down to 12 volts 6A/72 Watts should be able to handle anything that runs off 12 volts with a amps rating of no more than 6A and draws no more than the 72 watts. So if I have a mounting plate for both 18 and 40 volt batteries and as long as the amps and watts of the 12 volt appliance is between the range of the step down specs I should be alright? Do I have this right?

Would I need this converter on each appliance? Or just one for the system. Its conceivable that multiple things will be drawing power at once.

Would I need some sort of isolater switch that was eather automatic or manually switched before using the battery pack system?

[Drunkenclam]

Are you saying you want to use the cartridge batteries in your power tools (drill/saw etc)
to run the RV 12v systems? I suppose it might work, although hiw
long a battery would last running a 12v fridge, I suspect wouldn’t be for long, and may damage the battery (I’m no expert) but even when doing hardwork with a drill for a while the batteries can get a bit warm)

The step down convertor ratings are the max it can convert when given an adequate supply. I personally would give myself 10% leeway and not give a load of more than 5.5A.